This is the first time I've sewn a gusset of any kind, and it would never have occurred to me had it not been for the climbing jeans.

I ended up trying it on a sacrificial pair of trousers that I'd bought from the EVERYTHING £2 section of the secondhand shop, originally with the intention of ripping them up to scavenge material, zips and fastenings. They were too short in the crotch so they didn't even fit anyway. But now they may actually be usable.