2.1 Addressing, postal and land systems in Sierra Leone

Sierra Leone covers about 71,740 km² on the Atlantic coast of West Africa. The provisional results of the 2021 Mid-Term Population and Housing Census counted 7,541,641 people, a figure that was publicly contested [4]. English is the official language and Krio the language that almost everyone shares; Mende predominates in the south and east and Temne in the north and west, alongside Limba, Kono, Kissi, Fula, Susu and others. Digits are written the same way whatever language is spoken, and numbers are commonly said in Krio or English.

No postcodes. The UPU sheet, dated 07/2002, gives the example Mr. John Kamara, 7A Ross Road Cline, FREETOWN and no postcode [1]. We found no announcement of postcodes or of a national address system by SALPOST, the Ministry of Communication, Technology and Innovation or the National Civil Registration Authority up to October 2026 (the record is in docs/existing-systems.md).

Freetown’s Plus Codes. FCC describes a Plus Code as “a unique eleven-character code that acts like a postal code and street address”, and gives its own office at 17 Wallace Johnson Street as FQR7+HX [2]. A points-based valuation piloted in the city in 2019 depended on exactly this kind of property identification [5]. FCC’s register is not open: it is a rates register, and its owners, valuations and payments are confidential.

Boundaries in motion. The 2017 reorganisation created the North West Province and the Falaba and Karene districts, and raised the number of chiefdoms from 149 to 190 [6]. In 2025 the Minister of Local Government announced Cabinet approval for two further districts, Bandajuma and Kpanguma, and new councils for Freetown and the Western Area Rural district [7]. A code built on any administrative unit would have changed twice in a decade.

Census and land. The 2026 Population and Housing Census is scheduled to begin on 2 December 2026 [8]; it will produce enumeration areas, not addresses. The National Land Commission Act 2022 creates a Cadastral Index Mapping Department, and the Customary Land Rights Act 2022 governs the registration and demarcation of customary land [9], [10]. Survey plans use the Sierra Leone 1968 datum projected to UTM zones 28N and 29N [11]. An address is not a title, and the model keeps the two apart.

2.2 International practice

The UPU’s S42 standard defines the components of postal addresses and templates for rendering them [12], and ISO 19160-1 gives a conceptual model of address information, including the lifecycle of an address and its aliases [13]. Both describe how addresses are structured once they exist; neither says how to create them where they do not. The UPU’s white paper on addressing argues that an address is a public good and a precondition of postal, financial and civic services [14].

The most systematic evidence on creating addresses in African cities comes from the World Bank’s review of street-addressing programmes carried out between 1989 and 2004 in 52 cities of 15 countries [15]. In a sample of 21 cities that completed the work, 1.2 million doorways were addressed at an average cost of US$5.7 per doorway, and before the programmes only 8% of streets had names. The review also stresses that an address directory is useful beyond the post, for example to reconcile tax rolls, and warns that without maintenance an address system quickly becomes obsolete, especially where the city keeps growing. That warning motivates two of our choices: to start from names that people already use, and to make the register extensible without ever renumbering it.

2.3 Grid codes and why none fits alone

Open Location Code (Plus Codes) divides the globe into a hierarchical grid of base-20 cells [16]. A ten-character code names a cell of about 14 m by 14 m, and a code can be shortened by a nearby reference town. It is open, offline and already used by FCC. Its drawbacks as a home address are that it mixes letters with a “+”, that people copy it rather than remember it, and that in a dense quarter one cell can contain several homes (Section 6 measures how often).

National grid addresses have been adopted in the region. Ghana’s digital property addressing system, launched in 2017, assigns codes to cells of about 5 m by 5 m [17]. Such schemes give every point a code but, like Plus Codes, the code is not something people already say.

Purely numeric national identifiers are easy to dial but carry no meaning; codes built on districts are easy to say but are invalidated by redistricting. Section 7 sets out these alternatives with measurements on the Sierra Leonean data.

2.4 Requirements

From this review we derived ten requirements, in order of priority:

1. Remembered without effort: a place name people already use and a short number said in groups, obtained with one tap.
2. Unique nationally on its own: no district, chiefdom, town or phone position needed.
3. The compound and its gate: the unit is the plot or family yard; buildings are sub-units; navigation ends at the gate.
4. Official addresses first: where the state already addresses a property, its address is shown first, and every point is expressed on FCC’s grid.
5. No administrative boundary inside a code.
6. Works offline.
7. Stable: a code is never moved and never reused.
8. Places, never people.
9. Open and transferable to the state.
10. Sierra Leone’s own: the model and its engine are specific to the country.

4.1 The forms of a code

A compound’s code exists in a small number of written forms (Table 2). Let $P$ be the plate name of the compound’s area, $N$ its number (three to five digits, never starting with zero), $A$ the area’s five-digit index, $F$ the four-digit national identifier of the name, and $g$ the position of the compound’s gate. Then $$\text{remembered} = P \;\sqcup\; N, \qquad \text{plate} = P \;\sqcup\; \gamma(N) \;\cdot\; \sigma(g), \qquad \text{machine} = \texttt{SL-}A\texttt{-}N, \tag{1}$$ where $\sqcup$ denotes a space, $\gamma$ writes the number in the groups in which it is said ($482$; $27\,16$; $157\,27$), and $\sigma(g)$ is the short Plus Code of the gate (Section 4.10). The check digit $c = \operatorname{Damm}(F \,\|\, N)$ of Eq. (14) belongs to systems only: it is returned and verified by the interface, and never printed on a plate.

4.2 Areas

An area is a named place that people use, frozen at first issue, with no administrative parent. Its seed is an OpenStreetMap place node, either primary (city, town, suburb, quarter, village) or secondary (neighbourhood, hamlet, locality, isolated dwelling). Let $S_1$ and $S_2$ be the primary and secondary seeds and $d(f,s)$ the great-circle distance from footprint $f$ to seed $s$. Each footprint is assigned to $$\operatorname{area}(f) = \begin{cases} \arg\min_{s \in S_1} d(f,s) & \text{if } \min_{s \in S_1} d(f,s) \le 4\ \text{km},\\ \arg\min_{s \in S_2} d(f,s) & \text{else if } \min_{s \in S_2} d(f,s) \le 15\ \text{km},\\ \arg\min_{s \in S_1} d(f,s) & \text{otherwise.} \end{cases} \tag{2}$$

Two corrections follow. A primary area whose estimated compound count exceeds 4,500 is split: a footprint moves to the nearest secondary seed within 3 km whose own nearest primary seed is that area, when that seed is nearer than the area’s seed. An area with fewer than 20 compounds is folded into the area of the primary seed nearest its own seed, within 30 km. The estimate uses 0.779 compounds per footprint, the ratio measured in Freetown during the design. Areas never cross the Sierra Leone River estuary, so Lungi and the Bullom shore keep their own areas even where a Freetown seed is nearer across the water.

Of the 9,316 seeds, 5,184 areas hold homes (47 large areas were split and 1,466 small ones folded); 134 of them, with 192,608 compounds, lie in FCC’s area. The median area holds 93 compounds, the 90th percentile 604 and the largest, Makeni, 10,639 (Figure 2).

4.3 Plate names

The name in a code must be short enough to say with a number and to paint on a gate. A plate name $s$ is accepted when $$\operatorname{ok}(s) \iff \lvert \operatorname{NFC}(s) \rvert \le 12 \;\wedge\; \lvert \operatorname{words}(s) \setminus L \rvert \le 2 \;\wedge\; s \text{ contains no digit}, \qquad L = \{\text{of}, \text{the}, \text{and}, \text{on}, \text{at}\}, \tag{3}$$ where $\lvert\operatorname{NFC}(s)\rvert$ counts characters after Unicode normalisation (a hyphen or apostrophe counts as one). A name over the limit passes through fixed steps: a documented short form that people already use, if one exists (none has yet been documented for Sierra Leone); cleaning (“Community”, “Comm”, “Estate”, “Area” and bracketed text dropped; “Town”, “Village” and “Ville” dropped only at the end of a name still over the limit, because many names are said whole: Allen Town, Congo Town, Murray Town, and Kissy Town is not Kissy); a digit written as a word (“Badasuma 2” becomes Badasuma Two); and a landmark word (Junction, Wharf, Street, Corner, Quarry) dropped, so that Gbangbatoke Junction becomes Gbangbatoke. Nothing else is cut automatically. Where shortening would drop a place word or a person’s name, lose a qualifier (Upper Komende, Magbesseh 1 and 2) or produce the name of another place or of a well-known town (Moyamba Junction is not Moyamba), the area is referred to its council with up to three candidates (Table 3).

The 156 referred areas carry a provisional plate name, under which 84,311 codes were issued; a council may change such a name once, and the numbers do not change. The official name always remains an alias (208 aliases, none dropped), so that “Gbangbatoke Junction 1051” finds “Gbangbatoke 1051”.

4.4 The name key

Two areas share a national range when their plate names have the same key: $$\kappa(s) = \operatorname{join} \circ \operatorname{fold} \circ \operatorname{drop}_{\text{the}} \circ \operatorname{split} \circ \operatorname{punct} \circ \operatorname{casefold} \circ \operatorname{unaccent}\,(s). \tag{4}$$

The steps are listed in Table 4. The word folds reflect Krio and English spellings of the same word and the ways numbers are written.

The 5,184 areas carry 3,706 distinct plate names. Of these, 590 are shared, by 2,068 areas: Madina by 28 areas, Baoma by 24, Nyandehun and Sembehun by 23 each, and Moyamba and Looking Town by five each.

4.5 One national range per name

A name whose range has $d$ digits owns the numbers $10^{d-1}$ to $10^{d}-1$, which we cut into 90 units of $$u(d) = 10^{\,d-2} \tag{5}$$ numbers each: 10 for three digits, 100 for four and 1,000 for five. Each place $i$ of the name, with $n_i$ compounds, receives a block of whole units holding twice its compounds, $$k_i(d) = \max\!\left(1, \left\lceil \frac{2\,n_i}{u(d)} \right\rceil\right), \tag{6}$$ and the name takes the smallest length that leaves at least nine units in reserve: $$d^{*} = \min \Big\{ d \in \{3,4,5\} \;:\; \textstyle\sum_i k_i(d) + 9 \le 90 \Big\}. \tag{7}$$

Blocks are laid out from $10^{d^*-1}$ upwards, in the order of the places’ seeds along the national Hilbert curve of Section 4.6 (ties broken by area index); the rest of the range is the name’s reserve. A block that fills, or a new place with the same name, takes the next whole units of the reserve. When the reserve runs out, the name opens a range one digit longer (numbers of different lengths never collide), up to five digits. Blocks never move, shrink or get renumbered.

Worked example: Moyamba. Five places share the name, with 698, 4,313, 241, 35 and 26 compounds. At four digits (units of 100), Eq. (6) gives $14 + 87 + 5 + 1 + 1 = 108$ units, more than the 81 available; at five digits (units of 1,000) it gives $2 + 9 + 1 + 1 + 1 = 14$ units, which fit (Table 5, Figure 3).

Worked example: Masorie. Masorie Community in the Western Area Rural district (plate name Masorie, 48 compounds) and two villages called Masorie in Tonkolili (134 and 54 compounds) share a three-digit range: 100–199, 200–469 and 470–579. Masorie 151 and Masorie 315 are two places 42 km apart.

The resulting lengths are summarised in Table 6 and, by district, in Figure 4(b). The largest shared range is Kpetema’s: seven places with 11,008 compounds, at five digits. No name needs more than five digits, so none needs a qualifier.

4.6 Numbering within a place: the Hilbert curve

Compounds are numbered along one Hilbert curve [24] of order 16 over a fixed national frame, $\varphi \in [6.85^\circ, 10.05^\circ]$ N and $\lambda \in [-13.35^\circ, -10.20^\circ]$, evaluated at the centre of each compound’s main building. The Hilbert curve preserves locality better than other common space-filling curves [25], so neighbours receive neighbouring numbers. The cell coordinates are $$x = \left\lfloor \frac{\lambda + 13.35}{3.15}\, 2^{16} \right\rfloor, \qquad y = \left\lfloor \frac{\varphi - 6.85}{3.20}\, 2^{16} \right\rfloor , \tag{8}$$ and the position $h$ along the curve is accumulated over the bits $s = 2^{15}, 2^{14}, \dots, 1$: $$\begin{aligned} &r_x = [\,x \wedge s \neq 0\,], \qquad r_y = [\,y \wedge s \neq 0\,], \qquad h \leftarrow h + s^2\,\big((3 r_x) \oplus r_y\big),\\ &(x,y) \leftarrow \begin{cases} (s-1-y,\; s-1-x) & \text{if } r_y = 0 \text{ and } r_x = 1,\\ (y,\; x) & \text{if } r_y = 0 \text{ and } r_x = 0,\\ (x,\; y) & \text{if } r_y = 1, \end{cases} \end{aligned} \tag{9}$$ where $\wedge$ is bitwise conjunction, $\oplus$ exclusive or, and $[\cdot]$ the indicator. Figure 5 shows the curve of order 3. The $n$ compounds of a place with block $[\ell, u]$, sorted by $(h, \text{stable identifier})$ and indexed $i = 0, \dots, n-1$, receive $$N_i = \ell + i\,\delta + \left\lfloor \frac{\delta}{2} \right\rfloor, \qquad \delta = \left\lfloor \frac{u - \ell + 1}{n} \right\rfloor . \tag{10}$$

Worked example: Moyamba 15727. The compound’s main building lies at $\varphi = 8.1602399$, $\lambda = -12.4327947$. Eq. (8) gives $x = 19\,082$ and $y = 26\,833$, and Eq. (9) a curve position $h = 774\,683\,469$. Moyamba town’s block is 12000–20999 for 4,313 compounds, so $\delta = \lfloor 9\,000 / 4\,313 \rfloor = 2$, and the compound is the one with index $i = 1\,863$ in curve order: $$N_{1863} = 12\,000 + 1\,863 \times 2 + \lfloor 2/2 \rfloor = 15\,727 . \tag{11}$$

Figure 6 shows the result over the whole town: numbers rise smoothly across quarters, and a person who knows one code can guess the neighbourhood of another.

4.7 New compounds

A new compound whose curve position lies between those of its issued neighbours $a$ (at or before it) and $b$ (after it) takes the free number nearest the midpoint, strictly between them: $$N_{\text{new}} = \arg\min_{\,a < m < b,\; m \text{ free}} \left| m - \left\lfloor \tfrac{a+b}{2} \right\rfloor \right| , \tag{12}$$ with the edges of the block standing in for a missing neighbour. Because Eq. (10) leaves half of each block free, a gap normally exists; between Moyamba 15725 and 15727 a new house receives 15726. A full block takes a further block from the name’s reserve (Section 4.5). A point within 3 m of an issued entrance or outline, or inside an outline, is not issued a new code; the existing one is returned.

4.8 Decoding without context

The trailing digits of a written code are the number, whether typed in groups or not. A final single digit after a grouped number, if a system sends one, is the check digit. The name key gives the national range and the number gives the one block that holds it: $$\operatorname{decode}(P, N) = \big\{\, p \;:\; \kappa(P) \in \{\kappa(p)\} \cup \operatorname{aliases}(p),\;\; N \in \operatorname{blocks}(p) \,\big\}. \tag{13}$$

By construction the set has at most one element. A number in no block, or in a name’s reserve, is answered “no such address” and is never guessed into a nearby place. An official name that ends in a number (“Mile 91 1001”) is also tried with the number as part of the name, every split being tried, longest number first.

4.9 The check digit

The check digit uses Damm’s algorithm [26], [27], which, like Verhoeff’s earlier dihedral-group scheme [28], detects every single-digit error and every transposition of adjacent digits, but needs only one table lookup per digit. For the digit string $F \,\|\, N = t_1 t_2 \dots t_m$, $$c_0 = 0, \qquad c_k = T[c_{k-1}][t_k] \quad (k = 1, \dots, m), \qquad c = c_m, \tag{14}$$ where $T$ is the table of a totally anti-symmetric quasigroup of order 10: $$T = \left[\begin{array}{cccccccccc} 0&3&1&7&5&9&8&6&4&2\\ 7&0&9&2&1&5&4&8&6&3\\ 4&2&0&6&8&7&1&3&5&9\\ 1&7&5&0&9&8&3&4&2&6\\ 6&1&2&3&0&4&5&9&7&8\\ 3&6&7&4&2&0&9&5&8&1\\ 5&8&6&9&7&2&0&1&3&4\\ 8&9&4&5&3&6&2&0&1&7\\ 9&4&3&8&6&1&7&2&0&5\\ 2&5&8&1&4&3&6&7&9&0 \end{array}\right]. \tag{15}$$

Worked example. The name Moyamba has the national identifier $F = 2674$. For Moyamba 15727 the digits $2,6,7,4,1,5,7,2,7$ give the running values $$(c_1, \dots, c_9) = (1, 4, 9, 4, 1, 5, 5, 7, 0), \qquad c = 0 . \tag{16}$$

The transposition 15772 gives check 7, and the single error 15737 gives check 9. Because every place of a name shares the name’s identifier, Damm’s guarantee holds across them: a slip that lands in another Moyamba’s block is also caught (Section 6.4). The check digit is for systems: the interface returns it and verifies it when a system sends it, but it is never printed on a plate and people never type it.

4.10 Spot codes: Open Location Code

A spot code is the point’s Plus Code, exactly as the specification and FCC use it [16]. Five pairs of base-20 digits from the alphabet 23456789CFGHJMPQRVWX encode, pair by pair, a latitude digit and a longitude digit at resolutions $\rho_j = 20^{\circ}, 1^{\circ}, 0.05^{\circ}, 0.0025^{\circ}, 0.000125^{\circ}$. With $\Phi_0 = \varphi + 90$ and $\Lambda_0 = \lambda + 180$, $$a_j = \left\lfloor \frac{\Phi_{j-1}}{\rho_j} \right\rfloor,\quad b_j = \left\lfloor \frac{\Lambda_{j-1}}{\rho_j} \right\rfloor,\quad \Phi_j = \Phi_{j-1} - a_j \rho_j,\quad \Lambda_j = \Lambda_{j-1} - b_j \rho_j, \qquad j = 1, \dots, 5, \tag{17}$$ and the code is the sequence of characters $\alpha(a_1)\alpha(b_1)\dots\alpha(a_5)\alpha(b_5)$ with a “+” after the eighth, where $\alpha(k)$ is the character of index $k$ in the alphabet, counted from zero.

Worked example. For Moyamba 15727, $\Phi_0 = 98.1602399$ and $\Lambda_0 = 167.5672053$ (Table 7). The full code is 6CW95H68+3V, a cell of about 14 m by 14 m; an eleventh character (6CW95H68+3VW) narrows it to about 3 m.

The code is written short against one of 31 fixed reference towns, dropping its first four characters when the point lies within 0.4° of the town on both axes: 5H68+3V Moyamba. Recovery prefixes the town’s own first four characters and moves by one degree if the result lies more than half a degree from the town, the Open Location Code rule, so recovery is exact within 0.5°; the margin of 0.1° tolerates another application placing the town slightly differently. FCC’s office, FQR7+HX, recovers against Freetown to 6CW8FQR7+HX: the engine and FCC use the same code. Figure 7 shows one ten-character cell subdivided into its 400 eleventh-level cells, with the compounds whose entrances lie in it.

4.11 Compounds: grouping structures

The registry groups footprints into compounds before any number is given, one area at a time, after sorting them by stable identifier. Let $\operatorname{dist}(a,b)$ be the shortest distance between the outlines of footprints $a$ and $b$ (zero if one touches or overlaps the other), $|a|$ the area of $a$, and $\eta(a)$ the number of footprints in its 100 m × 100 m cell. A satellite $a$ ($|a| < 30$ m²) joins the nearest principal $b$ ($|b| \ge 30$ m² and $|b| \ge 2|a|$) when $$\operatorname{dist}(a,b) \le r(\eta(a)), \qquad r(\eta) = \begin{cases} 8\ \text{m} & \eta \le 25 \ \text{(village yard)},\\ 4\ \text{m} & 25 < \eta \le 70 \ \text{(town)},\\ \text{no grouping} & \eta > 70 \ \text{(dense)}. \end{cases} \tag{18}$$

Before this rule, a footprint whose centre lies inside a mapped closed wall, fence or hedge (40 m² to 20,000 m²) belongs to that enclosure’s compound, and estates, government, military and police compounds become parents whose inner compounds keep their own codes (Table 8). Only satellites join, and only principals receive them, so two houses are never merged. In Kroo Bay, Susan’s Bay and the slopes above Kissy there are no walls and yards are shared; merging there would send people to the wrong door, so the dense rule groups nothing.

Results. The 1,615,701 footprints form 1,359,282 compounds, 1.19 footprints per compound: 1,097,084 single buildings, 137,580 village yards, 70,435 town joins, 53,834 compounds in dense cells and 349 from mapped walls. 1,151,027 compounds have one structure, 169,990 two, 30,855 three and 7,410 four or more. With the 2021 census count, a compound holds on average about 5.5 people. Table 9 gives the figures by district.

Validation. We re-ran the rules on the 240 walls and fences in OpenStreetMap that enclose two or more buildings, with the walls hidden from the rules. 9.6% of the enclosures came out whole and 9.2% exactly, and a further 36.2% missed only one building. Inside a Freetown wall the buildings often stand apart while neighbours across the wall stand close, so proximity alone cannot recover walled compounds. The rules are deliberately cautious: merging two codes later costs nothing, whereas splitting a wrong merge would give a family a new code.

4.12 The building profile

Every structure carries a use class with its confidence and source, its size, its floors where known, and a status. The Sierra Leonean taxonomy includes block houses, pan body houses (Krio pan bodi, corrugated iron), mud houses, round huts, storey buildings, shops, markets, offices, schools, health facilities, places of worship, industry and mining, ancillary buildings and buildings under construction. Classes are inferred in a fixed order: OpenStreetMap amenity, shop, office or tourism tags (confidence 0.85); the building type (0.8 to 0.85, with the material for pan-body and mud houses); an N-Ways place inside (0.7, or 0.6 for two or more); the material alone (0.6); a very small (under 6 m², ancillary) or very large (at least 2,000 m², warehouse) footprint (0.35); otherwise unknown (Table 10).

96% of structures are unknown until people confirm them, because 94% of the country’s OpenStreetMap buildings carry only building=yes. OpenStreetMap gives floors for 1,120 buildings. A home never carries a name.

4.13 Freetown: the council’s Plus Code first

In FCC’s area (134 areas, 192,608 compounds) every card and plate shows the property’s Plus Code first, as FCC’s address (“FQQ6+9G Freetown”); the N-Ways code follows, to be said aloud and to distinguish the buildings of one property. The Plus Code shown is that of the compound’s main building, which is how FCC addresses properties. Where FCC’s register records a different code for a property, FCC’s code replaces it. FCC’s property identifiers are never shown publicly. If SALPOST, the Ministry or the National Land Commission introduces an official address elsewhere, the same rule applies: it comes first in its area, and the N-Ways codes remain valid beside it.

6.1 Uniqueness and decoding

Every one of the 1,359,282 machine forms occurs once, and no two compounds share a remembered form once names are compared by their key, Eq. (4). Decoding the written form of every code, exactly as printed on its plate, returns its own machine form in every case. The test suite of the reference engine also checks the frozen register for overlapping blocks and for any number that would decode to two areas; there are none.

6.2 Length

The number has three digits for a quarter of compounds and four for well over half. The remembered form, name and number together, has a median length of 12 characters and a 90th percentile of 16; 94.3% of codes have at most 16 characters. The longest codes belong to the 42 areas whose provisional names still exceed the limit (31,637 compounds, 2.3%); once their councils choose short names, no code will exceed 18 characters (a twelve-character name, a space and five digits, as in Looking Town 13316).

6.3 Coverage and official addresses

Every compound in the footprint data has a code. 192,608 compounds (14.2%) lie in FCC’s area, where the council’s Plus Code is the official address and is shown first; for the remaining 1,166,674 (85.8%) the N-Ways code is the only address at door level.

6.4 Slips across places of one name

The danger peculiar to shared ranges is a typing slip that lands in the block of another place with the same name. We sampled 40 numbers in every block of every shared name and formed every single-digit substitution and every transposition of neighbouring digits that stays within the name’s range and falls in another place’s block: 653,805 such slips. With the check computed over the name’s identifier and the number, Eq. (14), none is accepted, as Damm’s theorem guarantees. Had the check been computed over the area’s own identifier instead, the receiving system would compute the check for a different payload, and 71,342 of the slips (10.9%, about one in nine) would have been accepted.

6.5 Plus Code cells

Of the 1,359,282 compound entrances, 259,143 (19.1%) share their ten-character Plus Code cell with at least one other compound. A ten-character Plus Code therefore cannot serve as a home address on its own in Sierra Leone: in nearly one case in five it names more than one home. This is the main quantitative reason for printing the Plus Code under the name and number rather than in their place.

6.6 Worked examples

Table 12 lists real codes from the register: the compound nearest each of several well-known places.

A rider who is told “Moyamba, one five seven, two seven” types moyamba 157 27 and is shown one compound, in Moyamba District, with its gate. moyamba 22014 lies in the block 22000–22999 of a Moyamba in Kenema District: there is no list to choose from and nothing to guess. A partner system that sends Moyamba 157 72 with check 0 is refused, because the check of 267415772 is 7.