You can also vary the size and feel of items in each pocket since mostly you will be locating with only your hands. Many internal frame packs have only one pocket. This makes them more streamlined for squeezing through narrow canyons and fighting onto buses, but less convenient for getting to your gear. Pack makers offer optional accessory pockets which fit onto the pack, often by attaching to compression straps. Each pocket usually has two separate compartments.
Some packs can carry up to three extra pockets.
They add, however, considerable bulk and floppiness to the pack. Built-in pockets are more streamlined and stable. The only pocket I need or want on my toploader backpack is the built-in hood pocket. This is most apparent in packs with a bottom sleeping bag compartment. Here the relatively light and bulky sleeping bag forms a base for other items to ride on.