The sizes of the jumps in all international/open horse trials are closely regulated: I don't know the measurements in metric, but in feet and inches, 3ft 11in is the maximum height, measured from the point at which an 'average' horse would take off. You then have to allow for the fact that all the fences are fixed, are usually made of massive timber, may (and often do) have large open ditches underneath, may be on a slope, round a corner, into or out of water, emerging from dim woodland into sunlight or vice versa, and you begin see that although the size is not huge, the fence may still be extremely testing.
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