As you learn to ride, you will build one skill on top of the other. After you have mastered how to walk and halt your horse, you will probably move on to how to cue your horse to trot, and how to stay comfortable as the horse trots. If you're comfortable, your horse will be too, because you won't be bouncing up and down on its back. One way to take the bounce out of the trot is to learn to post. And as you master posting you'll be adding on yet another skill.
Not long after you learn how to post the trot you are going to hear your instructor say, "You are on the wrong diagonal!" This often causes no end of frustration for many new riders. Really, there’s something else to think about? Sometimes even experienced riders will meet a horse that has them double-checking their diagonals until they get accustomed to the horse’s way of going.
What are diagonals and why do they matter? Diagonals are a basic riding skill for both English and Western riders. Your horse really doesn't have diagonals--you do, and only at the trot. At the canter, you need to learn about leads. And horses have definite leads, so it’s not as much about what the rider is doing. There’s no way to put your horse on the right diagonal. You have to be on the right diagonal and that’s something you have to master on your own.
Diagonals help both horse and rider stay comfortable and balanced as they turn corners and go in circles.
How do you know you're on the correct diagonal? It has to do with the direction you are traveling. It doesn't matter as much if you're on the correct diagonal if you're on the trail, but if you're riding in an arena and turning lots of bends, it helps to know you're on the correct one. That doesn't mean diagonals aren't important out on the trail though.
Some horses seem to throw you on the wrong diagonal all of the time. If you find you're on the wrong diagonal, it's easy to change to the right one.
The Importance of Practice
The most important thing about diagonals at the trot is to keep practicing. You’ll learn to recognize which horses make you pick up the wrong diagonal, and how to compensate so you hit the right diagonal most of the time. Like any skill, it takes practice. Eventually, it will become second nature, and you won’t even have to check as you learn to ride by feel, rather than having to visually take stock of what leg you are rising to. In fact, there will come a time when riding on the wrong diagonal just feels odd.