If you’ve ever had to learn a foreign language, you may have struggled to memorize plural forms of nouns. German, for example, has about a half a dozen ways of forming plurals, and it’s a chore to remember which kind of plural each noun takes. English, by comparison, is ridiculously easy. Here’s how it works for nearly every English noun: add -s to the end. Sometimes you need to insert an e before the s, and sometimes you need to change a preceding y to ie, but that’s the rule in a nutshell.
Of course, there are still plenty of exceptions: a couple that end in -en (oxen and the strange double plural children), a handful of umlaut plurals (man–men, foot–feet, mouse–mice, etc.), some uninflected plurals (usually for domesticated or game animals, such as sheep, deer, and so on), and a plethora of foreign borrowings (particularly from Latin and Greek) that often follow rules from their donor languages but occasionally don’t. There are a few other oddballs—like person–people, for example—but nearly every English count noun fits into one of these categories.
But there’s one plural that doesn’t fit into any of these categories, because it’s been caught for centuries in a strange limbo between count nouns, which take plural forms, and mass nouns, which don’t. It’s dice. If you need a refresher, mass nouns generally refer to things that are not discrete, such as milk or oil, though some refer to things that are made of discrete pieces “whose indivual identities are not usually important to us,” as Arnold Zwicky put it in this Language Log post—words like corn or rice. You could count the individual grains or kernels if you wanted to, but why would you ever want to?
And this is how dice slipped through the cracks of language change. Originally, die was a regular noun that formed its plural by adding an s sound to the end. (For the moment, let’s leave aside the issue of spelling, because Middle and Early Modern English spelling was anything but standard.) At some point in the history of English, the final -s in plurals was voiceless, meaning that it was always pronounced with an s sound, not a z sound. But then that changed, probably sometime in the 1500s, so that the final -s was always voiced—that is, pronounced as a z—unless it followed a voiceless sound. Strangely, this sound change seems to have affected only the plural and possessive -s endings and not other word-final s’s.
But around that time, we start seeing the plural of die, when referring to those little cubes with pips used for games and whatnot, spelled as dice (and similar forms). In Modern English spelling, the final -s on a plural can be either voiced or voiceless, depending on the preceding word, but -ce is always voiceless. As the regular plural ending was becoming voiced for many many words, it remained voiceless in dice. Why?
Well, apparently because people had stopped thinking of it as a plural and started thinking of it as a mass noun, much like corn and rice, so they stopped seeing the s sound on the end as the plural marker and started perceiving it as simply part of the word. Singular dice can be found back to the late 1300s, and when the sound change came along in the 1500s and voiced most plural -s endings, dice was left behind, with its spelling altered to show that it was unequivocally voiceless. In other senses of the word, die was still thought of as a regular count noun, so its plural forms ended up as dies.*
Dice wasn’t the only word passed over in this way, though; truce (originally the plural of true, meaning “pledge” or “oath”), bodice (plural of body), and pence (a contracted plural form of penny) come to us the same way. Speakers subconsciously reanalyzed these words as mass nouns or singular count nouns, so their final s sounds stayed voiceless. Similarly, once, twice, and thrice were originally genitive forms, but they ceased to be thought of as such and consequently retained their voiceless sounds, respelled with ce.
But the strange thing is that whereas the words mentioned above made the transition to mass nouns or new singular count nouns, usage of dice has been split for centuries. We’ve never fully made the switch to thinking of dice as a mass noun, used regardless of the actual number of the things, because, unlike rice or corn, we do frequently care about the number of dice being used. Instead of a true mass noun, it’s become an uninflected count noun—one dice, two dice—for many people, though it exists alongside the original singular die. But singular dice is rare in print, because we’re told that it’s properly one die, two dice, even though some dictionaries note that singular dice is much more frequent in gaming than die.
So where does that leave us? You can go with singular die and possibly be thought of as something of a pedant, or you can go with singular dice and possibly be thought of as a little ignorant. As for me, I usually use singular die and feel twinges of self-loathing when I do so; I haven’t had the heart to correct my boys when they use singular dice.
*For more on the reconstruction of the plural ending in English, see the section on the English plural suffix in the chapter “Reconstruction” in Language History: An Introduction, by Andrew L. Sihler (Philadelphia: John Benjamins, 2000).