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Originally Posted by Siberys  This means you can take average on a 20 sided die. Average is 10.5 and rounded down, it's 10. It basically means you can take average or roll for a chance of perfection (20 being perfect). |
Also, the reason you can take 20 on some skills and not others is because it is assumed that you keep trying, failing many times, before acheiving the success you were looking for. For any skill that that has negative consequences for failure or does not allow retries, you cannot take 20.