stirrup question.

Ok so this is probably a really stupid question but I am at a loss. I just bought some new barrel racing aluminum stirrups. I got them home and went to put them on my saddle and realised one side of them was curved. Now I can't figure out which way they go on? Does the curved side go towards the front or back? They look just like the ones below:
http://www.statelinetack.com/item/polished-aluminum-barrel-stirrups/E001621/
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