Many styles of ascenders (the newer Petzl models being an exception) have two holes near the bottom. One is a big one that can fit a carabiner (or two), and the other is a small one that can’t usually fit a carabiner. This small hole can be used to tie a permanent tether from cord so you can’t drop your ascender, something more common on expedition-style climbing with a lot of fixed ropes. (Screen grab below from video about K2.)

Logical question: Why don’t ascenders have a slightly larger second hole so you COULD fit a small carabiner in it? I dunno! If anyone has the answer, please tell me.

If you're a big wall climbing instead of on an expedition, here’s an easy enhancement you can do to make this little hole more helpful: add a small quicklink. This makes a handy second point of connection for a carabiner.

• I use a Kong 4mm stainless steel link. Cost was about $6. These look pretty darn small, and are not rated or certified by any official CE of UIAA, but apparently the major access breaking load is 1400 kg, which is #SuperGoodEnough. These are a hard to find. I got mine at HowNOT2.store.

• A 5 mm stainless link works fine as well. It is of course slightly bigger, and takes a greater variety of carabiner shapes. It also might inspire a little more confidence, so feel free to use 5 mm if you like =^)

• Crank it down with pliers and add some Loctite thread locker if you have it.

• For ascending and cleaning an aid pitch: use the large hole to clip your tether with a locking carabiner, and the quicklink to clip your ladder with a non-locking carabiner.

Of course, there are some other options to attach your tether carabiner.

Newer Petzl ascenders have one large hole in the bottom instead of one large and one small. This lets you clip both your tether carabiner and your ladder carabiner into the same hole.

You could also clip your ladder carabiner to your tether locker, but I find having them separated with a quick link is cleaner and less prone to twisting and other weirdness.

The foundation of aid climbing is having simple, safe, and easily repeatable systems that you don't have to think about too much. This is one small way to do that. Every time you are getting rigged to clean a pitch, you know exactly where your tether and your ladders get clipped.

Or, if you use a Grigri for an ascending system, you can put a pulley or a carabiner on this quicklink to give yourself a downward pull and a little mechanical advantage when you’re ascending.

Here's the set up for for seconding / cleaning an aid pitch. Tether and locker to big hole, ladder and non locker to quicklink. Simple, clean, easy to check.

This is one of several ways to do it; give it a try and see if it works for you.