A six inch piece of old steel tape, cut from a broken retractable tape measure, can be carried in your wallet. This is a very handy tool for checking dimensions at the lumberyard or hardware store.
Another handy use for a broken retractable measure is to cut lengths of say, 10, 20, 30 cms and mount it to your tool box, work table or cutting tables.
If you don't have a broken retractable tape, you can always use a dollar bill, just remember -
Length = 6-1/4" - folded = 3-1/8"
Width = 2-5/8" - folded = 1-1/4"