A comma often serves to link two independent clauses, but it does so in partnership with a conjunction (as shown in this sentence). When a pair of independent clauses butt up against each other with a comma but no conjunction, the grammatical infraction is known as a comma splice—the name refers to the writer’s effort to splice the clauses together with a comma, which is the wrong use of the tool.
Which tool is the correct one? Several are available in your writer’s toolbox: The semicolon (a supercomma of sorts) is usually the best choice, but depending on the interrelationship of the clauses, you could, alternatively, employ a period (creating two distinct sentences) or even a dash (showing an abrupt break in thought). Here are three sentences containing a comma splice and their revisions.
1. “You can sense the frustration, their morale is down,” Smith said of his colleagues.
“‘You can sense the frustration; their morale is down,’ Smith said of his colleagues” is the technically correct solution, but a semicolon is overly formal for a quotation. In this case, use the more casual dash: “‘You can sense the frustration—their morale is down,’ Smith said of his colleagues.”
Another option is to break the statement into two sentences; but insert the attribution between them: “‘You can sense the frustration,’ Smith said of his colleagues. ‘Their morale is down.’” (Do not use a semicolon or a dash after an attribution.)
2. The devices themselves don’t use logarithms, rather they are tools to reduce multiplication and division of natural numbers to simple addition and subtraction operations.
A transitional word such as rather between two independent clauses is a clear signal that a stronger punctuation mark than a comma is required, because one should follow the word, and a stronger punctuation mark should set the clauses off from each other. A semicolon will suffice: “The devices themselves don’t use logarithms; rather, they are tools to reduce multiplication and division of natural numbers to simple addition and subtraction operations.” (Rather can also begin a separate sentence, but the second clause is so closely related to the first that a semicolon seems more suitable.)
3. The frame was finished two years later—a little behind schedule, the structure was supposed to open in 1989.
The inclusion of a dash in this sentence complicates it, because it introduces confusion about which phrase deserves the most emphasis. A simple solution is to demote the “Hey, look at me!” dash to a more workmanlike comma and use a semicolon to tack on the final phrase as an independent clause: “The frame was finished two years later, a little behind schedule; the structure was supposed to open in 1989.” One could use a dash in place of the semicolon, but the information that follows isn’t surprising enough to merit the attention-seeking punctuation mark.