A practical english grammar - Vyssaja skola 1978

Nouns in apposition
Sound symbols

Another way in which nouns modify other nouns is by apposition: two nouns of equal rank are put side by side, and the second explains, identi­fies, or is another name for, the first. In speech, the second noun or noun phrase is often spoken with pauses before and after, and this is shown in writing by commas. The absence of commas indicates the absence of pauses.

Charles,' King of England . . .  King Charles

My uncle, Fred Wallace, . . .  My uncle Fred

John, my older brother, . . .  My brother John

Quite often appositives appear in phrases introduced by of:

the city of Chicago

the science of biology

the art of painting