NEXT IN importance to the longest pause of all, the period or full stop, comes the shortest, the comma. The practice does not seriously differ from the theory implied by the etymology: comma, the Latin transliteration of Greek komma, related to koptein, to cut, means literally 'a cutting', hence 'a cutting-off', hence 'a part cut off', hence a clause, which, after all, is nothing but a part, especially a (comparatively) short part, cut off from the rest of the sentence; hence the sign that indicates the division. In modern practice, the comma serves to separate not only clauses but phrases and words; more precisely, certain kinds of clauses and certain kinds of phrase and certain groupings of words.
In modern usage, the comma subserves predominantly the grammar, the construction or syntax, of a sentence; formerly the comma indicated primarily the rhetorical pauses, as, quite often, it still does. To attempt a rigid dichotomy of rhetorical and grammatical uses of the comma would be crassly stupid: and this condemnation, as we have already seen, applies to punctuation in general.
Although the separation, whether of single words or phrases from other single words or phrases, or of single words or phrases from clauses, is, on the whole, more modern than the separation of clause from clause, it is easier to treat the comma in the following apparently arbitrary, yet practical and convenient, order:
commas between single words:
(a) nouns or pronouns
commas between word-groups (other than phrases) apprehended as units
commas between single words and word-groups