Designed to carry items such as  a wallet, cell phone, power bar, emergency tools, come

in a variety of sizes.  They either velcro to the seat rails or attach with a quick release

type clip.  Some models are expandable to allow for a little more storage if needed for

a longer ride for example.  All seat bags come with some sort of reflective material, and some

will come with a place to clip on a safety light.

Voyager Expandable Seat Bag

This seat bag among many others expands

to allow for that little bit more storage

for when you need it.  A seat bag such

as this one expands from 1.2 litre to 1.7 litre

capacity.