: It's the custom or a It's a custom to do something

[Rezafo]

Part of a dialogue in New Interchange book 3 unit 5:

Marta: Yes, but what do you do when you are invited to someone’s house here?

Karen: well, it’s the custom to bring a small gift.

I don’t know why I feel that it should be “ It’s a custom to … “

Why “the” and not “a” is preceding custom?

[Raymott]

Part of a dialogue in New Interchange book 3 unit 5:

Marta: Yes, but what do you do when you are invited to someone’s house here?

Karen: well, it’s the custom to bring a small gift.

I don’t know why I feel that it should be “ It’s a custom to … “

Why “the” and not “a” is preceding custom?

It's a custom of that place/society. But it's the custom for when you're invited somewhere.
Marta wants to know what the specific custom of that place is for when you're invited somewhere; and Karen tells her what the specific custom is.
Karen could have said, "It's a custom to bring a small gift, " but Marta has told her what specific custom she wanted to know about, so Karen uses "the".

[Exuberant Verbosity]

The words "a" and "an" indicate indefinite articles, while the word "the" indicates definite articles.

It's a custom ... (not a definite / specific custom)

It's the custom ... (a definite / specific custom about that situation)

Bring me an egg ... (any egg will do as it's non-specific / indefinite article)

Bring me the egg ... (a specific egg is required as a definite article is used)

[Tdol]

Isn't the actually the definite article and not an indicator of a definite article?

[Exuberant Verbosity]

Isn't the actually the definite article and not an indicator of a definite article?

In terms of function, yes it is. Yet, the, a and an are always placed before nouns thereby specifying 'things' (whether tangible or abstract). Thus, it denotes whether a 'thing' / 'article' is specific (definite) or non-specific (indefinite).

I don't separate the article (the, a , an) from the noun or qualified noun as the two work together.

What do you think?

[Raymott]

In terms of function, yes it is. Yet, the, a and an are always placed before nouns thereby specifying 'things' (whether tangible or abstract). Thus, it denotes whether a 'thing' / 'article' is specific (definite) or non-specific (indefinite).

I don't separate the article (the, a , an) from the noun or qualified noun as the two work together.

What do you think?

Maybe I'm misunderstanding, but "the, a and an are always placed before nouns thereby specifying 'things' (whether tangible or abstract)" sounds wrong.
To me, this could mean two things i) all nouns are preceded by "the, a and an" or ii) "the, a and an" are always followed by a noun.

As a counterexamples to i), I'd cite plurals ("I like apples" - the zero article), and abstract terms - 'happiness, love, life, beauty ..." ; and nouns preceded by other determiners such as "these, those, that, this ..."
As a counterexamples to ii), I'd cite nouns which are preceded by adjectives: "the big brown dog." I'll concede on this point that they are eventually followed by nouns.

I might also have misinterpreted 'things' (whether tangible or abstract).
Are you saying that nouns always specify things, and hence all abstract nouns refer to things? Or are you only referring to some abstract nouns, which happen to be things, and hence take an article?

I guess it must work for you in class, but I like the idea of a noun having a grammatical independence from its article.