The hand is an ancient, nay archaic, unit of length, which can be traced back to Ancient Egypt. As far as Britain is concerned, the hand was standardised to four inches – that is, the approximately breadth of a man’s hand, including the thumb – by King Henry VIII in 1514.
Nowadays, most European countries measure the height of horses in metres and centimetres but, curiously, Britain and Ireland still do so in hands, despite the hand being obsolete for any other purpose. The height of a horse from the ground to the tallest point on the body, a section of the spinal column above and just behind the shoulders, known as the ‘withers’.
The average height of a thoroughbred racehorse is sixteen hands and one inch at the withers, which is usually written as ’16.1hh’; note that while hands are expressed decimally, they are base four units, so a horse measuring 66”, or 5’6”, would be described as ’16.2hh’, not ’16.5hh’.