“ It is familiar in the titles, showing the colour of their wands of office, of the gentlemen ushers of the three principal British orders of knighthood, the ushers of the Garter and St Patrick being “Ushers of the Black Rod,” and of the Thistle “Green Rod.” The use of a rod as a measuring implement has given rise to the use of the word for a measure of length = 51⁄2 yds. or 1621⁄2 ft.; this length is also named a pole or perch, the origin of the application being the same as in “rod”; as a measure of area, a rod = a square pole or perch, 301⁄4 square yds.= 2721⁄4 square ft., 160 rods = 1 acre. ”