Frilled: Frilled shorts are not for everyone, but if you are a bit concerned that your derriere doesn't look as round or as curved as you'd like it, frills along the back of the short will definitely plump that area up in a delectable way.
The boy brief or boy short also eliminates the derriere seams of traditional panties, but can create awkward lines around the thighs; sometimes this result can make the legs look like stuffed sausages.
Coincidentally, that also meant they were focusing on "...every taut derriere muscle of each female Olympic beach volleyball athlete," according to Theresa Thomas of the The Catholic Exchange.
Keep in mind that the strings and elastic bands used in these swimsuits give you a bit of leeway in the size you choose, so sizing is actually more about how much derriere you want to bare.
Thongs provide less coverage than a panty and sacrifice the back portion of the panty in order to remove any visible lines that are created by the derriere seams of underpants.