"OPPOSED Rod, pole, and perch are used interchangeably (though in practice rod is most often used) in measuring the 16-1/2-foot *length*, but for area the use of ""perch"" is almost exclusive, probably stemming from the perch also historically representing units, if I recall correctly that defined both Scots and Irish miles and acres, whereas this didn't apply necessarily to pole, and rod, pretty certain the pole never referred to Irish and Scots units.  

"