The comma is in the family of stops, punctuation marks which include the semicolon,colon and full stop. For me in modern poetry there are no rules as such but there is sensible use of and a purpose for commas.
Commas are used to imply a completion of sense, when the sense changes a comma might be used. It can also suggest a pause or an emphasis on a word or clause.
It can also affect the pace of the rhythm and the clarity of a line or clause. These two examples show how each poet has used commas to slow down the pace, take a breath and move on into new meaning.
Where I have dipped to drink again, to be
Faithful to the admonishment on her cup,
Remember the Giver, fading off the lip.
Seamus Heaney from A Drink of Water
defines the surface,
so we are drawn to it,
and yellows glittering
like a stand
of beech trees late
in Germany, in fall.
Jorie Graham from Two Paintings By Gustav Klimt.
** The form of Jorie Graham's poem has been changed.