Most of the United States west of the original thirteen colonies is described not by metes and bounds but by a rectangular grid: townships six miles square, each divided into 36 sections of one square mile, each of 640 acres. The system was designed to be readable without any measurement, and it largely is — which is why a legal description can be four short groups of letters and numbers and still identify a parcel to the acre. This guide explains the layers and how to read them.
| Unit | Size | Area | Where it sits relative to |
|---|---|---|---|
| Township | 6 miles × 6 miles | 36 square miles = 23,040 acres | North or south of the baseline |
| Range | 6 miles wide | — | East or west of the principal meridian |
| Section | 1 mile × 1 mile | 640 acres | Position within the township |
A principal meridian is a north-south reference line, and a baseline is an east-west one. There are more than thirty principal meridians across the country, each with its own name — the Sixth Principal Meridian, the Gila and Salt River Meridian, the Boise Meridian — and the name is part of a complete description, because "Township 2 North, Range 3 East" means nothing without knowing which meridian and baseline it is measured from.
A description such as T2N, R3E, Sec 14 unpacks as follows: a township two tiers north of
the baseline, three ranges east of the principal meridian, and section 14 within that township. The
layers are read from large to small, which is why a full legal description reads like a string of
nested boxes and ends in the smallest one.
| Layer | Approximate area | Written as |
|---|---|---|
| Township | 23,040 acres | T2N, R3E |
| Section | 640 acres | Sec 14 |
| Half section | 320 acres | N ½ |
| Quarter section | 160 acres | SE ¼ |
| Quarter-quarter section | 40 acres | NE ¼ SE ¼ |
| Government lot | Irregular | Lot 4 |
The 40-acre quarter-quarter is the smallest unit with a common name, and it is why "the back forty"
entered the language. It is read right to left: NE ¼ SE ¼ is the north-east quarter of
the south-east quarter of the section, which is a 40-acre square in the north-east corner of the
south-east quarter.
Sections are numbered in a boustrophedon — an ox-plough pattern — starting at the north-east corner and running west across the top row, then east across the next row, and so on until section 36 finishes in the south-east corner. The result is that sections facing each other across a common boundary always differ by one, which makes it easy to see at a glance whether two adjacent parcels share a line. It also means the numbering is not a simple left-to-right raster, which is a persistent source of errors in datasets built by people who assumed it was.
The grid assumes a flat, square world, and the Earth is neither. Lines of longitude converge towards the poles, so a township laid out as six miles square is very slightly trapezoidal, and the discrepancy accumulates as you move north. The system deals with this by surveying the south and east boundaries of each township first, subdividing into regular sections, and then letting the remaining error fall into the north and west tiers.
Those residual strips are the government lots, and they are irregular by design — narrower than a standard section, sometimes markedly. A description that says "Lot 4" instead of a quarter-section is telling you that the parcel is one of these fit-in pieces, and its area has to come from the recorded plat rather than from arithmetic. Treating a government lot as if it were a quarter-section is a straightforward way to be wrong by several acres.
The system covers most, not all, of the country. The public land survey system applies to roughly thirty of the fifty states. The original colonies and the states created from them — plus Texas, and parts of a few other areas — were never laid out this way and remain in metes and bounds, described by bearings and distances from monuments. A description in sections and ranges outside the PLSS states is a sign that something has gone wrong upstream.
The chain units are not a coincidence: the grid was laid out with a 66-foot Gunter's chain, and 80 chains to the mile is exactly why a mile is 5,280 feet and why a section is 640 acres rather than some rounder metric number.
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