What does hh mean?
hh means hands high. In everyday writing, 15.2hh, 15.2 hands, and 15.2 h are usually pointing to the same height format.
Convert hands, inches, and centimeters for horse listings, sale ads, and buyer notes. Hands are not decimals: 15.2hh means 15 hands plus 2 inches, not fifteen point two.
Example: 15.2hh
One hand is four inches. The digit after the point is the remaining inches, so it can only be 0, 1, 2, or 3. That is why 15.4hh is not a valid horse-height format.
The unit has an old practical origin: people measured horses with the width of a hand before modern measuring sticks made the system exact. Today, hands high is standardized to four inches and measured at the withers, the high point between the shoulder blades.
For a listing, write the height as hands first, then add centimeters or inches if it helps your audience compare quickly.

hh means hands high. In everyday writing, 15.2hh, 15.2 hands, and 15.2 h are usually pointing to the same height format.
Many people use 14.2hh, about 148 cm, as the pony line. Competition rules can be more specific, including whether the pony is measured with shoes.
If your audience may use metric, write both: 15.2hh (157.5 cm). It keeps the listing readable for local buyers and easier to compare for everyone else.