Horse glossary
Diagonal
A posting-trot timing check (your instructor will teach you how to spot it).
What it means
In the posting trot, your diagonal is the pair of the horse's legs you rise with, and riders choose it to match the direction they're going. On the correct diagonal, you rise as the horse's outside front leg moves forward and sit as it comes back to the ground. The word "diagonal" also names a straight line across the arena from corner to corner.
Why it matters
Posting on the correct diagonal helps the horse balance on turns and circles, because your weight comes down as its inside hind leg steps under. Changing diagonals when you change direction also keeps the work even on both sides.
Example
Ask your instructor to call your diagonal.
Try this
Glance down at the horse's outside shoulder without dropping your head: you should be rising as that shoulder swings forward. To change diagonals, sit for one extra beat, then rise again.
Common mistake
Many riders forget to change their diagonal when they change direction. Make it a habit every time you cross the arena or switch the direction of a circle.