Percentile dice rules are insane

It was recently pointed out to me that I misunderstood the rules for percentile dice rolls in D&D 5e and Pathfinder 2e. I quickly made my way through the first two stages of grief—denial and anger—and am now firmly stuck in the bargaining phase. Surely people don't think this is a good idea, do they?

The consensus on the internet is that my interpretation makes no sense, that anyone that shares my perspective is a total fucking idiot, and that the rules as written couldn't be more reasonable. This consensus is so overwhelming that I cannot help but feel a deep sense of paranoia; never in the history of the world has the internet been in such unanimous agreement with itself, and therefore I can't help but suspect some sort of global conspiracy in the implementation of these insane percentile dice rules.

On the surface of it, the rules sound very simple and reasonable. There are two dice; one regular d10, with possible values of 1, 2, 3, ..., 0; and one "tens" (percentile) die, with possible values of 00, 10, 20, ..., 90. Roll both dice, and combine the numbers. But how exactly do we interpret the numbers?

There are two examples given in the 5th edition D&D Player's Handbook. The first shows values of 70 and 1 on the percentile and d10, respectively. 70 + 1 = 71. Easy!

It then goes on to do the—for want of a better word—"math" for the following values: 00 and 0. They boldly state that 00 + 0 = 100. If we don't think about this too carefully, that seems reasonable. But I put it to you that 00 + 0 should actually equal 10, and that treating 00 as either 0 or 100 is an egregious affront to dice, mathematics, and indeed to the physical world itself.

I have four main objections to the rules as written:

• These rules fundamentally change the meaning of a d10.
• Schrodinger's die: With these rules, a 00 on the percentile die simultaneously means both 0 and 100; the waveform collapses—the true value is revealed—only when you roll the d10.
• These rules are the only time that dice can behave as arbitrary symbols, rather than as mathematical functions.
• These rules are the only time that you can roll a zero on a die.

Let's look at these objections in more detail.

When a d10 is not a d10

The percentile rules besmirch the good name of one of the most iconic dice, needlessly turning it into an aberrant abomination masquerading as a d10.

Every single time you roll a d10 in D&D and Pathfinder (and probably every mainstream TTRPG ever developed), the possible values range from 1 to 10, with the 10 usually represented by a 0 on the die. Sorry, that is, every single time except for when you roll it with a percentile die using these diabolical rules.

When we say "Roll a d20", it's implied that there are 20 possible values: every integer from 1–20, inclusive.

When we say "Roll a d100", it's implied that there are 100 possible values: every integer from 1–100, inclusive.

But when we use a percentile die and a d10 to make a d100 roll—with rules as written—the d10 actually behaves like some ungodly chimera comprising a d9, a d10, and a d10–1.

It's a d9 in the sense that the maximum value of the roll is 9; it's a d10 in the sense that there are 10 possible outcomes; and it's a d10–1 in the sense that a) there are ten possible outcomes, but b) the maximum value is 10–1, that is, 9. This is deranged, and entirely breaks the mental model of a d10 (and indeed, any dice that we use in TTRPGs) that is so consistent everywhere else.

Why must we foster an identity crisis by redefining what it means to be a d10? Surely it is easier to be consistent, and say that a value of 0 on a d10 represents the number 10, everywhere and every time we roll it? Why do we need this one exception?

Schrodinger's 00

If you roll a 00 on the percentile die, that can represent either 100 or 0.

If you roll 1–9 on the d10, then we treat the 00 as 0; so, for example, 00 + 5 = 5.

However, if you roll a 0 on the d10, then that 00 on the percentile die inexplicably transforms into 100. The exact same die roll—00— can mean two completely different things; it is either zero or one hundred.

This is absolutely, irrefutably bonkers. I simply cannot believe that we, as a community, think that it's okay to introduce quantum mechanical principles into our dice rolls.

Dice are mathematical functions

This follows on from the previous objection, that is, that the percentile die can mean two different things, depending on the value of the d10 that is rolled.

Every other time you roll a die (you can sense a theme here), the possible outcomes range from some low integer (usually 1) to some higher integer (4, 6, 8, 10, 12, 20). When you roll multiple dice, you add the numbers together. 2d10 means roll two d10s, and add the two numbers together. This is so intuitive that it barely warrants an explanation. The outcomes of a dice are represented by integers; when rolling dice, we add the resulting integers together to get the final result.

But with percentile dice rules-as-written, we do not. 0 + 00 does not equal 100. Even if we do the sane, consistent thing, and treat a 0 on the d10 as representing 10, then 0 + 00 still does not equal 100.

You can roll a zero on a die

I'm running out of steam here, but... where the fuck else do you roll a dice and get a value of zero?

With these rules, a 0 on the d10 means 0, not 10 (as it is everywhere else). Again: This is the only time you will ever roll a die in D&D or Pathfinder and have zero as a possible result.

The sane way to roll percentile dice

My proposed alternative is simple: Treat a d10 like a d10, and treat a percentile/tens die as a percentile/tens die. A 0 on a d10 means 10, as it does literally everywhere else. A 00 on a percentile die always means 0, as should be readily apparent by the two zeroes on the die.

Therefore, 00 + 0 = 10.

This is consistent and logical and doesn't require us to invoke quantum mechanics to interpret percentile die. We can rest assured that the dice behave in a consistent manner, ranging from 1 to some positive integer; we know that all is right in the world, and that to interpret the outcome of multiple dice rolls, we simply add the results together.