What are these?
 

Two of my bikes have a little knob thing on the inside of the rear triangle on only the side with gears and derailleur. It's about three inches up from the rear "corner" or the triangle. One of my other bikes has a tab thing that has a threaded hole on the gear side only. It is a couple inches up from the rear "corner" of the rear triangle as well. The tab extends forward. Can someone please tell me what these are for. Thanks.

The knob is for hanging the chain on when you remove the rear wheel. The threaded tab looks like an attachment point for a chain guard, like my mother's bike had (1930s).

the first pic looks like a threaded mount for a chainguard,the second is definatly a chainhanger. used to hang the chain on when you remove the wheel while the bike is upright to facilitate quick changes, or while on a stand.

It's a chain peg... When you take the rear wheel off, you can hang the chain on it if you wish while the rear wheel is off.

Jay

Thanks for the help everyone. You are most definitely right, because the tab is on a commuter type of bike (Koga Miyata Road Gentleman). So I can imagine some might want a chain guard on that type of bike. There is no chain hanger on that bike. I guess it is because it has a sachs 3 x 7 hub and there will likely never be a quick change. I've been calling these things bafflers because they baffled me. So nice to know what they are for. I'm just learning and really appreciate this site and everyone (well, almost everyone) on here.