Two Left Socks
My spouse recently ended up with two left toe socks and no way to wear them, because each one had lost the only sock that matched it. My drawer, and now our kids' drawers, cannot have that problem: the kids and I wear one style of sock for every occasion, so any two make a pair. Our socks are fungible. Shoes have good reason to come in lefts and rights, but socks do not need that complication, and neither do most of the systems I build. Do not build systems that need a specific matched pair when a part that fits anywhere would do.