chain
Lengthexact by definition
A chain is exactly 66 feet, 22 yards, or 20.1168 metres. It is the unit the American landscape is laid out in: ten square chains make an acre exactly, eighty chains make a mile, and the Public Land Survey System's section corners were set by dragging one.
Watch out: A cricket pitch is one chain long, which is the easiest way to picture it. Engineers' chains of 100 ft also existed and appear in older railway and highway stationing, so a chainage figure from an old drawing needs its chain identified.
| 1 ch | 20.1168 m |
Edmund Gunter designed it around 1620 to reconcile two incompatible systems. At 66 ft it is a whole number of the traditional units and, because it holds 100 links, it also decimalises cleanly. That single compromise is why an acre is 43,560 ft² rather than something round.