You measure how wide something is by assessing its width. When you check how long it is, you have its length. Then you measure how tall it is, and you get…height. But not “heighth.”
The “-th” ending comes from Old English. It’s also used in other Germanic languages as a way to change a verb into a noun. Once you start looking for it, you’ll find that it’s easy to find in modern English too, in words like growth, stealth, birth, death, oath, health, and even bath. But as common as it is, the -th ending used to be used even more often. A couple of centuries ago (or more) verbs like “blow” and “spill” had noun forms “blowth” and “spilth.”
And sure enough, back in the 1700s “height” was “heighth” — sometimes spelled “highth”. But if you listen instead of read, you’ll discover (to your utter astonishment, I’m sure) that although “heighth” ended in “th,” it was pronounced as if it ended in “t.” This was the common pronunciation for “-th” endings at the time, but “width” and “length” kept their “-th” sound because “dt” and “gt” are pretty hard to say.
“Heighth” also looks odd to English readers; the “ghth” at the end is an unusual string of consonants to put together. So after the 1700s the spelling tended to standardize around “height.” But in fact the pronunciation “heighth” is still around; there are regional dialects in North America and England where everyone says “heighth” with the ending “th” even though they don’t spell it that way. And once in a while you’ll still see “heighth” just as Charles Dickens used it in Great Expectations: “Pip, I wish you ever well and ever prospering to a greater and a greater heighth.”
