[RPG] Why do most people consider a “00” and a “0” on a percentile roll a 100


Assuming the results of a percentile roll ranges from 1-100, why would the results of a "00" and "0" roll be read as a "100"?

In any other context outside of a percentile roll, the "0" on a d10 is interpreted as a "10". Also, the "00" is read as a "0" in every other roll of the d100. But specifically for "00" and "0", it results in a "100".

This means that the possible results of a "00" roll with any roll of the d10 is 1, 2, 3, 4, 5, 6, 7, 8, 9, and 100.

This raises an additional question of why a "0" on a d10 in the context of a percentile roll is treated as a "0".

Is this simply to make the reading of the percentile roll easier, or simply to make it so a 100 wouldn't require an unaesthetic "90" and a "0" instead of the more visually-appealing "00" and "0"?

Best Answer

In common practice a d100 is effectively a 0-99 roll, with the stipulation that 0 be treated as 100

The game needs a way to roll 1 to 100 with equal chances, and no chance of getting zero.

Let's start by just looking at how we are set up to roll the results from 1 to 99, and then we'll get to the special case of getting 100.

In order to have a practical way to get the numbers 1-99, we roll a double-digit and a single-digit d10 together, and add them arithmetically. We add the two numbers we literally see, so a "90"and a "3" is 93; a "00" and a "3" is just 3; a "90" and a "0" is just 90, etc.

Two problems initially: we have no way to get 100, and we have the possibility of getting a total zero by rolling "00" and "0".

Solution to both: count the "00" and "0" combo as 100, instead of zero. With this simple adjustment, we have exactly what we want: a 1-in-100 chance of getting all possible results from 1 to 100, and never getting a final zero.

Admittedly, it is a matter of convention

Understandably, you normally treat a "0" on a regular d10 as "10", and you could continue to do so even when using it in d100, such that if you rolled "80" and "0" and treat the "0" as 10, then you get 90. But game communities tend to converge on one common practice or another, and the one outlined above is the one that has taken hold. But technically, you could use either method, as long as everyone at the table understands and agrees.

A final historical note

In the old days, few players had two d10s to enable rolling a d100 all in one go (maybe because dice sets were more expensive relative to income back in the day). We'd roll our single d10 for the tens-place, and then pick it up and roll it again for the ones-place. This made the zero-substitution rule very exciting and suspenseful! If my initial one-die roll was zero I'd be thinking, "Dang, I probably will end up with just a 1-9, but I've got a shot at a 100!". And everyone around the table would be thinking the same thing, and would watch with tense anticipation what the second roll was going to be.

This dynamic added some fun to the game (that you don't "feel" when rolling two d10s), and might help explain why the method "stuck."