At 135 cubic inches, the seat bag is large enough to hold your wallet, keys, bike tools, etc. It will even fit most standard size water bladders. The bag slips over the back or bottom of the seat and can easily be removed and carried by a handle on the top. It has a zipper closure under a protective flap. This bag is very durable, puncture resistant, flexible, and water repellant. It provides excellent use of free space and will not interfere with the commuter bag, head rest, rear rack, trunk pack, or panniers. Compatible with TerraTrike seats. Not compatible with wide seats or TerraTrike versions 1.9 and older.