Bah.

• Decide to spend the day on which housemates are at work fixing the dripping bathtub faucet.
• Turn off water.
• Partially disassemble cold water faucet.
• Take faucet stem to Tags and ask what I should get.
• Salesman says "replace the washer here. It's a size 1/2" large, which we seem to be out of. (To digress, what sort of a numbering system goes 3/8, 3/8 large, 1/2, 1/2 large, 5/8, 5/8 large in sizes?)
• Go to Home Depot and buy 1/2" large washers and O ring thing.
• Come home. Swap washer and O ring.
• Turn on water. Note that faucet is now running rather than dripping.
• Turn off water. Wish there were a closer shutoff than basement.
• Decide this isn't actually a 1/2" large, it's a 5/8".
• Go back to Tags. They do have 5/8". I am briefly confused by the 5/8 large, as I had until then thought that "large" was a different diameter (thickness?).
• Come home. Swap washer again.
• Turn on water. Note that faucet is dripping more than it used to, though not running.
• Turn off water.
• Put the old O ring back. It was probably thinner than the new one.
• Turn on water. Note that it's dripping about as much as it used to.
• Wonder if it's actually the hot water faucet causing all the trouble.
• Turn off water.
• Partially take apart hot water faucet stem, but fail to be able to get the final bit out. (Interesting. I didn't take these two pieces apart at all in the cold water faucet. I didn't even know they *came* apart!). There's no way to get at the washer.
• Put hot water stem back together.
• Turn on water.
• Note that faucet is still dripping. Sigh.