The suffix is “teen”, yes? So why is 18 spelled “eighteen” and not “eightteen”? Is there a grammar rule governing double T’s?
There are no grammar rules.
We use -teen instead of onety-one, onety-two, onety three, etc. as is done for twenty and above, and that we use thirteen instead of threeteen and fifteen instead of fiveteen, etc.
You think there are strict and simple rules that need to be followed? 
It’s whatever people want it to be; spelling and grammar evolve by consensus.
Though in this case, it comes from adding -teen (or, rather, -tyne] to ehta. “Eight” did not have the extra “t” until after “eighteen” was coined.
(emphasis added)
Grammar is nothing but rules.
More to the point, however, there has never been any logic to English spelling conventions.
I hate to tell you this, but ehta and eight each have exactly the same number of ts in them.
That doesn’t look like “eight” getting an extra “t”, but rather “ehta” losing the final “a”. When that “a” went, the two t’s in “ehta-teen” merged into one “t”.
We drop the ‘ve’ in “five” and replace with ‘f’ to get “fifteen.” Why not drop the ‘t’ from “eight” to eliminate a silly extra letter.
nott that i know.
I’ll vote for this. There is no logic to English spelling conventions. Even the famous “i before e except after c” is fancied to sink under its own weight.
Why does friedo have Charter Member under his name twice?
Charter Member
Charter Member
When you have a two syllable word, and vowel in the first syllable is short, you will find a double consonant after that vowel. Like in “willow”. If it were written “wilow”, it would tend to be pronounced “WIE-low”.
Consider “Tiger” vs “Tigger”.
There are exceptions, of course, like “sugar”, but that’s also a loan word. I can’t think of any words where the consonant is double, and the vowel is long.
If we wrote “Eighttenn”, it would tend to indicate that the vowel sound before it would be short-- something like “AH-teen”. Of course, we have the utter bastardization of the vowel sound “eigh” which has it’s own problems, but still, the doublet would be a very odd spelling in English.
What really bugs me is that it is “forty” and not “fourty.”
In one of my programming classes we had a project to write a simple parsing program that translated numbers to their word representations and back (i.e. 112 <-> one hundred twelve). The biggest error by far? Half the people wrote “fourty” for “forty.” It was so bad they actually rewrote the test cases to accept “fourty” and only take off a point for the first occurrence of the error, rather than have everybody completely fail every test case where “forty” came up :smack:.
Can I suggest a perusal of Bill Bryson’s Mother Tongue?
It’s a discussion of the English language with a bias towards the perceived idiosyncrasies of English and the differences to American English.
Plainness
Vainness
Zealless
etc
There’s a bunch more, but those are in-line with the OP’s question about a stem word+suffix that results in a double consonant.
I don’t think it implies anything of the sort. “Ei” (or “eigh,” really) being shortened to “ah” wouldn’t even occur to me as a possibility as a native English speaker. To me, it’s that the consonant cluster “ghtt” in “eightteen” looks stupid, so “eighteen” it was.
Okay, iuf you write out “12th” using only letters, how do you spell it?
th?
Indeed, the t in eight has a strong pedigree, both in modern related languages (eight is acht both in Dutch and German, and in both languages the -t- is retained for eighteen, making achttien and achtzehn. But it also shows up in the romance languages (otto, huit, ocho), going back to ‘octo’ in latin.
twelfth? Is this a trick? have a been wooshed? why do you want to know how to spell twelfth?