Suppose that we have a simple sentence: "Our family lives in an old derelict house."
Do we need to put a comma between the adjectives "old" and "derelict"? I would be glad if someone explains me the appropriate rule for this case.
The conventional rule is that if two adjectives both modify the noun, then you put a comma between them. But if an adjective modifies the following adjective, you do not put a comma between them.
It is an old, woman's hat.
This is a hat that is old, and that is for a woman. The hat is old. The age of the woman is unspecified. You could have an old hat for a young woman.
It is an old woman's hat.
This is a hat that is for an old woman. That is, it is the woman that is old, not the hat. You could have a new hat for an old woman.
However, even in the first case, where both adjectives modify the noun, if there is unlikely to be confusion the comma is often left out.
So I think technically, you should say "a derelict, old house". But if you omit the comma, it doesn't make sense to say that the oldness is derelict, so there's no ambiguity in saying "a derelict old house".
In this case, you will be well understood with or without the comma.
However, the adjective order would seem more natural if you wrote
derelict old house without a comma OR used a comma (
old, derelict house).
The comma creates a slight pause in reading and can be helpful when putting the adjectives in a nonstandard order, which you might want to do if you're trying to emphasize one aspect over another or show closer connection between this sentence and one before or after it that emphasizes the closer attribute. For example (fiction):
Our community was settled hundreds of years ago, and little has been updated since. Our family lives in an old, derelict house. Half its shutters are missing and the front door swings in the breeze, barely attached by half a hinge.
Adjective order is not that easy and is somewhat flexible. This unsourced answer clearly puts quality before age, while this sourced answer does not have an explicit category for "quality" but puts judgment/attitude adjectives before age, with an explicit example of
a rustic old ... cottage. Having a
derelict old house is a straightforward application of the example.
Grammar Girl explains the rule as a difference between cumulative and coordinate adjectives, where cumulative adjectives modify the noun to form a new concept which is then modified by the adjective before to further modify the concept. In the case of cumulative adjectives, the comma is omitted.