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1.
Beads put on a string that are held in place by friction, and can only be moved when slid up or down the string purposefully by the user.
These have been used for centuries, now they are used by militaries and mountaineers to judge the distance they have hiked.

*How to use*
You have two sections, an "upper" section with 6 beads, and a "lower" section with 7 beads.
You count roughly 60-70 paces, however many paces it takes for you to go 110 meters. Every 60-70 paces, drag a bead down from the lower section. Every lower bead represents 110 yards, or 1/16 of a mile, hiked.
Every upper bead represents 880 yards, or 1/2 a mile.
When you've brought down every lower bead and reach up to bring down the non-existent 8th bead, bring down an upper bead, and reset the lower beads. Start all this over.

You can judge distance hiked over a flat surface to about a 10% error ratio. Remember if you're hiking over hills or something it'll mess this up.
Look around, they're all over military surplus sights.
by June 22, 2005