A run-on is a sentence in which two or more independent clauses (i.e., complete sentences) are joined without appropriate punctuation or conjunction, generally considered a stylistic error, though it is occasionally used in literature and may be used as a rhetorical device; a run-on is a comma splice, in which two independent clauses are joined with a comma without an accompanying coordinating conjunction, but some prescriptivists exclude comma splices from the definition of a run-on sentence. This does not, however, imply that they consider comma splices to be acceptable.
The mere fact that a sentence is long does not make it a run-on sentence; sentences are run-ons only when they contain more than one independent clause. A run-on sentence can be as short as four words—for instance: I drive she walks. In this case there are two independent clauses: two subjects paired with two intransitive verbs. So as long as clauses are punctuated appropriately, a writer can assemble multiple independent clauses in a single sentence; in fact, a properly constructed sentence can be extended indefinitely.
It is hard to argue that a run-on sentence is incorrect if it forms part of the direct speech of a real or imagined person, as the punctuation of speech is usually simply conjecture (except perhaps for a question mark after a definite question). For instance, if the first example below is direct speech (as it appears to be) it makes little difference if there is a space, comma, semicolon, or full stop between "gas" and "we", except perhaps that a space or comma might imply a faster rate of talking than a semicolon or full stop.
- A run-on sentence, without any punctuation or conjunction between "gas" and "we":
- My car is out of gas we cannot reach town before dark.
- A comma splice, which is considered a run-on sentence in English by some usage experts:
- It is nearly half past five, we cannot reach town before dark.
- Use separate sentences. However, this may disconnect related independent clauses and cause some of the meaning to be lost:
- It is nearly half past five. We cannot reach town before dark.
- Use a semicolon. This maintains the connection between the clauses while ensuring a pause between the two ideas:
- It is nearly half past five; we cannot reach town before dark.
- Use a coordinating conjunction with a comma.
- It is nearly half past five, so we cannot reach town before dark.
The short story ""Repent, Harlequin!" Said the Ticktockman" is known for its use of improper grammar, one example of which is a paragraph about jelly beans composed almost entirely of run-on sentences.