Punctuation of parallel structures
In parallel structures, punctuation depends on the number of elements in the string.
- Two elements take no commas!
xxx and yyy
rain or snow; advertising and marketing staff; bought and sold
- Three or more elements: comma after each element. A phrase conjunction (and / but / or) is required before the last element. In fact, the conjunction and marks the last element in the parallel structure.
xxx, yyy, and zzz
soil, minerals, and water;production, trade, and distribution of goods
aaa, bbb, ccc, or ddd
buy, sell, or trade commodities
Various punctuation marks have different "power," with the period being the most powerful sentence divider, followed by the semicolon and comma. In some contexts the semicolon and a conjunction + a comma can have the same power:
The United States is inhabited by 5% of the world population, but it uses roughly 25% of the world's commercial energy.The United States is inhabited by 5% of the world population; it uses roughly 25% of the world's commercial energy.
The comma, possibly because of its relatively small dividing power, has a large number of uses. It can set off prepositional phrases, sentence transitions, elements of parallel structures (words or phrases alike), subordinate clauses, or short simple sentences when boosted by a conjunction. It is in part due to the comma's flexibility that L2 writers find the punctuation rules dealing with commas confusing.