What should men wear at a Grand Prix dressage competition as participant?
You can look up the rules online at the USDF or USEF, and you can Google videos of riders.
Why are you asking? Generally by the time this is a practical question, you will have lots of trainers around to give guidance. Or are you wanting facts for a creative writing project or visual arts? That’s fine too! However give us a clue how horse savvy you are, because we could use a lot of confusing shorthand.
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I will say that men and women dress pretty much the same in dressage.