Oh, hands or 'hands high (hh)' is a very fun obscure unit of measure. It is only used for measuring the height of horses ... and nothing else.

One hand is 4 inches, and the number after the decimal is additional inches above the last 4 measured. So 10.0hh, 10.1hh, 10.2hh, and 10.3hh are all valid measurements, but 10.4hh is just 11hh, and 10.5hh or higher is complete nonsense. And since horses might raise or lower their heads in varying degrees at any time, making it very difficult to get a consistent measurement, the measurement is always taken at the point of the withers (shoulders) -- basically the highest point on a horse's back, at the base of their neck -- because that measurement doesn't change much depending on how the horse moves. If you want to be really precise about the measurement, it should be taken while the horse is standing still with all four hooves standing squarely and flat on level ground.

And, yes, it actually is still in common use whenever people talk about the size of a horse.