Functional Tattoos

process writes: While considering what to tattoo where I started thinking about tattoos that actually serve some function. A quick google came up with this site suggesting a ruler on your arm, identification or medical information. I am however not completely satisfied with these options, so I'm wondering if the slashdot crowd can come up with some suggestions for functional tattoos. It doesn't really need to be useful, as long as it has some use (like a connect-the-dots tattoo), although something that has some value other than entertainment would be interesting aswell. A quick brainstorm came up with things like a star map, a solar dial or some other useful information or figure — or maybe even a (very) useful and hard to remember mathematical formula. I'm sure there are plenty of ideas for information that you just won't to leave home without.
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