When should an introductory prepositional phrase be followed by a comma? When should it not?

Use a comma to separate a group of preposition phrases of more than four words when the phrases come at the beginning of a sentence.

Do not use a comma between separate phrases unless they are in a series.

A comma may also set off a single prepositional phrase at the beginning to make the sentence clear. A comma is recommended after any introductory prepositional phrase of more than four words.

Correct: Under the kitchen table the dog cowered.

Correct: Under the spreading chestnut tree, the village smithy stands.

Correct: Under the pile of clothes, we found his wallet.

Incorrect: On the sand, of the beach, by the inlet, we relaxed in the sun. Correct: On the sand of the beach by the inlet, we relaxed in the sun. Correct: Over hill, over dale, we hit the dusty trail.