The quad anchor is an increasingly popular choice for many climbers. (See a detailed article about the quad here.)

The standard way of tying it, with a doubled 180 cm or 240 cm sling, works great, provided you have that gear with you. However, those slings are unusual sizes, and can be hard to find.

What if you want to make a quad anchor and all you have is a pair of 120 cm slings? No problem. Put them together, tie two bight knots, and you’re good to go. Tip: Try to tie the bar tack sewing in one of the two loops (above, it's in the left.)

In the photo, notice we have a figure 8 on a bight rather than an overhand knot. This has a couple of benefits:

1. It uses up a bit more cord, which makes your master point higher, which is usually more ergonomic

2. The extra bend in the figure 8 makes it easier to untie after its been loaded

The photo shows a newer Metolius 11 mm Dyneema sling, paired with an Edelrid Aramid cord sling (one of my favorites for anchor building.) Strong anchor, simple (easy to inspect) completely redundant, fast to tie, zero extension if a sling were to fail, and a pair of (pretty darn well) equalized connection points. Bonus: using the blue Aramid cord it makes it much easier to untie as well.

If the bolts are very close together, you might be able to tie a quad anchor with a one 120 cm sling. However, doing this takes a fair bit of fiddling to get the knot and bar tack in exactly the right place, so I much prefer to use a 180 cm sling, or this method.