Note about measurements.

Length

1 rod = 5½ yards

4 rods = 1 chain (=22 yards)

10 chains = 1 furlong (=220 yards)

8 furlongs = 1 mile (=1760 yards)

Area

"a, r, p" figures refer to (a)cres, (r)oods* and (p)erches.

40 perches = 1 rood

4 roods = 1 acre

640 acres = 1 square mile

Think of a perch as 1 rod x 1 rod (5½ yards x 5½ yards = 30¼ square yards)

Think of a rood as 1 furlong x 1 rod (220 yards x 5½ yards = 1210 square yards)

Think of an acre as 1 furlong x 1 chain (220 yards x 22 yards = 4840 square yards)

*Confusingly, roods are also sometimes called rods.