I have seen the question from the title and its variants asked at various places and have decided to write this text and keep it as a reference. All standard backpack loops and their purpose are described.
There are at least four different groups of loops on a typical backpack. You will find them on the lid, on the bottom, on the front, on the shoulder harness, and sometimes also on the sides.
You can use them to attach a lot of gear that normally does not go into the pack. This includes trekking poles, an ice axe, skis, a helmet, a tripod, an axe, a riffle, an extra day pack, and some other items.
All of them are described below and shown in pictures. Note that text is not about straps, these are described in detail in my separate text.
Ice axe attachment loops
This attachment system includes a thick loop on the bottom of the pack, sometimes two, and accompanied with a tie-off bungee higher on the front of the pack. You can see this in the picture below from my Osprey Stratos 24 pack. The loop is on the left and the bungee is on the right.
You can use the same loop for trekking poles of course, but some packs have them separately, more below. If this loop is long enough you can attach a classic axe as well.
Note that these and other loops are the main difference between hiking and standard backpacks as discussed in my separate text.
Trekking poles attachment loops
Most packs come with some sort of attachment system for trekking poles. In a great design you will have them two as in the picture below, you can see them on the right.
In this pack they are paired with a single bungee tie-off higher on the pack; here you can see it on the left. But some packs do not have such upper attachment element, and in such cases you will use your upper side strap.
But in a number of Osprey packs they have their own unique Stow-on-the-Go attachment system. You can see it in my Osprey Stratos 24 pack, and I can tell you I use it a lot and I like it.
It includes a thick loop with a plastic cover that is on the left side of the pack, you can see it on the right in the picture, and a bungee cord that is on the left shoulder strap, this is the yellow cord on the left.
This attachment system is useful when you need free hands to take pictures, or to have snacks on the go, or when you have a scramble section in front of you. You can see how this works in this short video below.
Lid attachment loops
Typically you will see four separate loops as in the picture below from my Deuter pack. So with some piece of cord you can attach lighter items here. This can be a sleeping pad or anything similar.
Some packs have two separate short daisy chains with the same purpose.
Shoulder harness attachment loops
Here you can have at least two types.
1. Loops for water hose routing. You will find these on most packs, and usually on both sides of the shoulder harness. The picture below shows them in my Osprey Stratos 24 pack.
2. Some packs may have a loop for glasses. This is a frequent feature of Gregory packs. The picture below shows this in the Gregory Kalmia packs:
Some extra loops
- You will find packs with additional loops on the front. These are usually to attach a helmet holder. You have this feature in Deuter Futura Air Trek packs.
- But some larger packs have such loops to attach a smaller day pack from the same brand. You have such packs from Osprey like Aether & Ariel, Volt & Viva, and also packs from the Porter, Farpoint 80, and Sojourn series.
- There are packs with additional larger and stronger loops for skis. You have this in the incredible UNLTD packs from Osprey, see it in the Osprey UNLTD AntiGravity 64 Pack for Men & Women.
In summary, a hiking or backpacking pack usually has many loops from outside. Some of them can be used for a direct attachment of gear, and for some you have to add a piece of cord.
Some of the loops are also used in combination with straps described in my separate text. This is typical for bottom loops and side straps combination used for poles or an ice axe. In my separate text you can read also how to use them to attach a sleeping bag to a day pack.
Thank you for reading. Let me know if you think I missed to mention something important, there is a comment box below.