Measure phrases are special noun phrases that we use to explain how long or big or heavy or expensive something is: The programme was ten minutes .
The walk was five miles .
The meal was twenty pounds .
The word is only three letters
These measure phrases all include a number, like one or seven and a noun afterwards.
In the examples above these measure phrases are Complements of the verb.
You will notice that the nouns are all in the plural, as we expect.
We can also use measure phrases like these to modify nouns: a ten minute programme.
a five mile walk.
a twenty pound note.
a three letter word.
Here, these measure phrases are modifying the nouns programme , walk and note .
They are in the same position that we find adjectives in.
When we use measure phrases in this way, the noun in the measure phrase is not plural.
We see no S on the ends of the words in the measure phrases.
Really this is not very surprising.
Why?
Well, when we use a noun to modify another noun, we don't usually use plurals (there are exceptions of course).
So we usually say: a book collector an ant eater a cherry tart
We don't say: *a books collector *an ants eater *a cherries tart
ell.stackexchange.com/users/8084/Araucaria - Not here any more.
You might mean 'two-letter words'.
Be aware that there's a real hyphen between 'two' and 'letter', with which we don't use plural from of the latter word because such a hyphened phrase is used as an adjective, not a noun.
For example: I went on a trip of four days after my three- day work was done.
A boy of five years and another six- year -old girl.
ell.stackexchange.com/users/51868/iBug
The measure phrases, such as "two-letter", act as adjectives.
We do not pluralize adjectives in English.
Any accurate translation is an equivalent: any phrase that gets across the same meaning, or as close to that meaning as possible.
We should never primarily try for a word-for-word translation as our primary goal.
I don't have the "reputation" to comment on others' posts, but let's improve on sentences such as "The meal was 20 pounds.
"
No, the meal cost 20 pounds.
(More precise, easily translated, and enjoyable to read.)
ell.stackexchange.com/users/51884/R Wm
For a combination of 2 reasons: English has a Germanic grammar, and in Germanic languages, "twoletterword" is one word – a noun!
It does not matter that English writes it in 3 words (and as a result, calls it a noun phrase , not a noun) –
Germanic languages are older than writing.
As far as I know, all other Germanic languages would write it as "twoletterword", which may better represent how they work.
Inflecting a word's constituents is undefined in English.
You can only inflect the word as a whole, which is indistinguishable from inflecting the last constituent word (because Germanic compound words read like domain names – most significant part last, which is also where the inflection is).
ell.stackexchange.com/users/51993/user2394284
A compound noun combines multiple nouns to make a new noun, and treats the first noun as an adjective to describe the second noun.
Two letters and word are combining to make a new noun.
A car salesman is another example.
The true noun here is salesman.
The word car is there to tell us what kind of salesman they are, so it's being used like an adjective and therefore should not be pluralized like you might a noun.
Many compound nouns have no spaces between their constituent nouns, such as keyboard, dishwasher and bathroom.
Those examples demonstrate the sheer power of compound nouns--the ability to just take a singular noun and tack it on to another to make a new word.
Awesome.
And imagine how funny it would sound if you had to add s
/es to the first of the two nouns (keysboard, disheswasher, bathsroom...).
ell.stackexchange.com/users/51889/Rowan M
The issue is an understanding of the implicit but unspoken content:a 'letter' means an "instance of the letter object'a 'word' means a collection of letter objects Thus
a "two letter word" means two instances of the letter object combined in a wordwhile a word of 20 letters is a collection of size 20 containing letter objects
The English language recognises that the two usages describe different things and therefore have different structures
ell.stackexchange.com/users/52037/Rs3
Why is "letter" not plural in "two letter words"?  For me it's very strange as the equivalent in French would be plural but my English friend finds it totally normal.
Why is "letter" not plural in "two letter words"?
ell.stackexchange.com
<singular-vs-plural><attributive-nouns><measure-expressions>
ell.stackexchange.com/questions/123894
ell.stackexchange.com/users/51867/Gudradain
When measure phrases are used in this way, the noun in the measure phrase is not plural. Adjectives are not pluralized in English.  The noun in the measure phrase is not plural 
Measure phrases are special noun phrases that we use to describe the length, width, weight, or cost of something. The noun in the measure phrase is not plural when we use measure phrases in this way. Measure phrases, such as "two-letter", act as adjectives. In English, adjectives are not pluralized. Any precise translation is an equivalent: any phrase that conveys the same meaning, or as close to it as possible. 
A compound noun is formed by combining two or more nouns to form a new noun. Many nouns, such as keyboard, dishwasher, and bathroom, have no spaces between their constituent nouns. Compound nouns combine a singular noun with another to form a new word.






 The problem is that people don't understand the implicit but unspoken content of the English language. A 'word' is a group of letter objects, and a 'two letter word" is a collection of size 20 containing letter objects.