Irish Perch

Seven yards, or twenty-one feet, against the English perch of five and a half yards.

This is the unit that makes the Irish mile long. At seven yards against the English perch’s five and a half, it is just over a quarter larger, and everything built upon it inherits the difference: the Irish chain, the Irish furlong, the Irish mile of 2,240 yards and the Irish acre of 7,840 square yards. The system was used in Irish land surveying and appears throughout estate maps and the records of the Down Survey, and it persisted alongside English measures for long enough that Irish documents routinely had to specify which was meant. Ordnance Survey mapping of Ireland in the 1830s required the two to be reconciled parish by parish, a task that consumed a good deal of the survey’s effort and produced conversion tables that surveyors carried for decades afterwards.