The term parallel structures simply refers to similar grammatical forms. The forms can be words, phrases, clauses, or even sentences. Coordinating conjunctions (cc) always link parallel structures. The coordinating conjunctions are and, or, but (which are always coordinating conjunctions), and so, for, yet (which can be coordinating conjunctions but are also used in other ways). Also see cc (coordinating conjunctions) and / / (parallelism) in Correction Symbols Two.
Here are some examples of sentences containing parallel structures:
Mary owns a house and a car. (nouns)
Her house is white, gray, and green. (adjectives)
She takes good care of her house and of her car. (prepositional phrases)
Her house is old, but her car is new. (independent clauses)
Below are three rules (A, B, C) for punctuating parallel structures:
A. Two independent clauses can be separated by 1. a period, 2. a semicolon, or 3. a comma and a coordinating conjunction:
1. He sings. She dances.
2. He sings; she dances.
3. He sings, and she dances.
The comma is optional if at least one of the clauses is very short (five words or less), so this is also correct:
He sings and she dances.
Note that two independent clauses cannot be separated by 4. a comma only or 5. nothing.
4. * He sings, she dances. (not correct)
This error is called a comma splice, which is one kind of run-on sentence. See cs and r-o in Correction Symbols Two.
5. * He sings she dances. (not correct)
This error is called a fused sentence, which is another kind of run-on sentence. See fs and r-o in Correction Symbols Two.
B. Two of any parallel structures other than independent clauses are separated by a coordinating conjunction only:
Fred and George want to see Mary. (nouns)
Mary got in her car and drove away. (verbs)
I donít know where she is or when she will return. (subordinate clauses)
A comma is rarely used between just two parallel structures. Only a coordinating conjunction is used between two parallel structures (unless they are independent clauses as in A. above).
Exception: a comma is sometimes used between two adjectives which precede a noun. For example:
It is a dark, ugly room.
C. Three or more of any parallel structures, including independent clauses, are separated by commas and one coordinating conjunction before the last item:
1. xxxx, yyyy, cc zzzz
2. wwww, xxxx, yyyy, cc zzzz
3. vvvv, wwww, xxxx, yyyy, cc zzzz
4. uuuu, vvvv, wwww, xxxx, yyyy, cc zzzz
Note that, when there are three or more parallel structures, a coordinating conjunction (cc) comes before the last item. Also note that the final comma (the one before the conjunction) is optional. The parallel structures can be anything: nouns, verbs, adjectives, adverbs, prepositional phrases, subjects, predicates, subordinate clauses, independent clauses,...
Here are some examples of C.1. above (3 parallel structures):
He sings, she dances, and everybody claps. (independent clauses)
He won the lottery, quit his job, and bought presents for all his friends. (predicates)
He bought a car, a motorcycle, and a computer. (nouns)
The motorcycle is red, white, and blue. (adjectives)
Everybody wants to meet him, to talk to him, and to get some money from him. (infinitive phrases)