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The acre is an imperial unit of area.

One acre is equal to an oblong measuring $1$ chain by $1$ furlong.

That is, measuring $4$ rods, poles or perches by $10$ chains.

\(\ds \) \(\) \(\ds 1\) acre
\(\ds \) \(=\) \(\ds 4\) roods
\(\ds \) \(=\) \(\ds 10\) square chains
\(\ds \) \(=\) \(\ds 160 = 4 \times 40\) square rods, poles or perches
\(\ds \) \(=\) \(\ds 4840 = 22 \times 220\) square yards

Historical Note

The acre is a traditional measure of land arising from the days of ploughing in mediaeval times and earlier.

It was defined as the area of land that could be ploughed by $1$ ploughman with $1$ ox in $1$ day.

It is noted that an acre is not actually defined as a square area, but as an oblong strip $10$ times as long as it is wide.

The reason is readily apparent: its length is, as is apparent from its definition, one furlong, that is, furrow-long.

Linguistic Note

The word acre derives from the Old English æcer, meaning open field.

It is pronounced ay-ker, and was at one time spelt aker.