PS - only tangentially relevant to the original question "Why do you use two words for only one meaning?"
"Tie", as a noun (and also as a verb in some cases, but let's stick to nouns), has several meanings; here are just a few:
something you wear
a feature of the construction of railway tracks
a meeting between two sporting teams
a sporting match that ends with both sides scoring the same
the ending of a race, when two people have run equally fast
[these last two are obviously closely related, but not the same]
.
.
.
+ others I'm sure. In this case, Am English
can distinguish one meaning with the prefix "neck" (but doesn't always, as Ouisch has pointed out).
In another case there are three meanings competing for two words:
Am English:
Titled: = having as a heading (a book or paper)
? = bearing a title, such as "Sir" or "Lord"
Entitled = [?] Having a moral or legal right to
[as you can see by the ??, I'm not an authority on these meanings
]
Br English:
Entitled = having as a heading (a book or paper)
Titled = bearing a title, such as "Sir" or "Lord"
Entitled = Having a moral or legal right to
Since there are very few "titled" people in the USA (in the Br E sense), it makes sense for Am., usage to use the two words titled/entitled to divide up the remaining two meanings. The social background is not so clear-cut in Br English, so we're left with the uncomfortable situation of two words for three meanings.
To add to the confusion, many modern speakers of Br English (particularly academics) have - in the last 20-35 years, I'd guess (I first heard it in 1974, but that was in the mouth of a heavily American-influenced English speaking Dutch academic, and it sounded at the time very odd) - started to adopt the Am E usage (and even to
"correct" the traditional Br English use of 'entitled' (don't go there, it's a sore point

).
Anyway, this is just another example of how AmE and BrE often have different meanings for similar-looking words. I have never gone to work in my pants, though American men do it all the time. However, on very hot days, colleagues of mine have come to work in their shorts - something Americans would never do.
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