The use of commas in compounds, the third in the 3 Fundamental Comma Rules guide, appears as number 13 on the list of “The Top Twenty,” discussed in the previous post. This list of twenty most common errors made by college-age students, also reflects common errors made by writers of any age.
In fact, the failure to use a comma correctly when joining compound sentences results in two errors, the run-on or fused sentence (joining or fusing two sentences without a comma and conjunction) and the comma splice (joining two sentences with only a comma), which Martha Kolln in her book Rhetorical Grammar cites as among the most common of all punctuation error that writers make. (These two errors, the comma splice and the run-on sentence appear as errors numbers 15 and 16 respectively in the Top Twenty list.)
As we saw with the earlier discussion of the first fundamental comma rule, the use of a comma in a series, commas can function as separators; they occur between units that are co-joined, such as items in a series: red, white, and blue.
With compound sentences, a comma can be used to join two or more independent clauses. Independent clauses are clauses made up of subjects (or noun-phrases) and verbs (also called predicates or verb phrases). Sentences have been defined as strings of words that are punctuated with terminal punctuation and begin with a capital letter—something we all recognize. A sentence has also been defined as an independent clause that gives a sense of completeness.
With compound sentences, these clauses are equal in rank or importance.
To join these two compound sentences, the traditional rule is to use a comma and a coordinating conjunction. A comma is not strong enough on its own to join them.
There are just 7 coordinating conjunctions which makes them easy to remember, especially because the first letter of their name forms a convenient acronym: FANBOYS.
For, And, Nor, But, Or, Yet, or So (FANBOYS)
The relationship between the clauses is determined by the choice of the coordination conjunction. Examples:
The cat sat on the mat, and the dog slept on the rug. (equal)
The cat sat on the mat, but the dog slept on the rug. (contrast)
The cat sat on the mat, so the dog slept on the rug. (causal)
As stated above, a comma alone is not strong enough to connect the two independent clauses; it needs the support of a conjunction.
However, sometimes when the two independent clauses are short, the comma is omitted in creative writing as a matter of style and rhythm:
The rain beat down and the wind howled.
If a comma is used alone, it creates what is called a comma splice because the two independent clauses have been co-joined or spliced together with a just a comma (Top Twenty error #16.)
Comma splices may occur when some writers are tempted to leave out the conjunction because it adds a “certain flabbiness.”
If a writer wishes to combine two independent clauses without conjunction, the option is to use a semicolon which is strong enough to connect two independent clauses. This also produces a brisker pace.
Other writers are tempted to leave out both the comma and conjunction, but this is incorrect. It results in a fused or run-on sentence because two sentences have been fused or run together (Top Twenty error # 15).
In compound sentences, both a comma and a coordinating conjunction are needed. They act as a signal to let “the readers to know that another complete sentence is coming” (Kolln, Rhetorical Grammar 48).