The semicolon represents a division in thought somewhat greater than that represented by a comma, and somewhat smaller than that represented by a period. It may represent grammatical separation and logical connection at the same time; that is, it may indicate that two statements are separate units in grammar, and are yet to be taken together to form a larger unit of logic or thought.
[92a.] The semicolon is used between coördinate clauses which are not joined by a conjunction. (For a possible exception see [91b].).
- Wrong: He was alarmed in fact he was terrified.
- Right: He was alarmed; in fact he was terrified.
- Right: He drew up at the curb; he leaped from the car.
[Note.]—Very often the writer may choose freely between the semicolon and the period; in such instances the use of the semicolon implies greater logical unity between the clauses than the use of the period would show. Unless this logical unity is distinct, the period is to be preferred.
[b.] The semicolon is sometimes used between coördinate clauses which are joined by a conjunction if the clauses are long, or if the clauses have commas within themselves, or if obscurity would result were the semicolon not used. (Otherwise, see [91a].)
- Right: Very slowly the glow in the heavens deepened and extended itself along the eastern horizon; but at last the bright-red rim of the sun showed above the crest of the hill.
- Right: He arrived, so they tell me, after nightfall; and immediately going to a hotel, called for a room.
- Confusing: She enjoyed the dinners, and the dancing, and the music, and the whole gay round of fashionable life was a delight to her.
- Better: She enjoyed the dinners, and the dancing, and the music; and the whole gay round of fashionable life was a delight to her.
[c.] The semicolon is used between coördinate clauses which are joined by a formal conjunctive adverb (hence, thus, then, therefore, accordingly, consequently, besides, still, nevertheless, or the like).
- Wrong: We have failed in this therefore let us try something else.
- Right: We have failed in this; therefore let us try something else.
- Wrong: He was tattered and muddy, besides he ate like a cormorant.
- Right: He was tattered and muddy; besides he ate like a cormorant.
[Note 1.]—If a simple conjunction like and is used in the sentences above, a comma will suffice. But a comma is not sufficient before a conjunctive adverb like therefore. Conjunctive adverbs may be clearly distinguished from simple conjunctions (See [91a]). They cannot always be easily distinguished from subordinating conjunctions (see [90b, Note]), but the distinction, when it can be made with certainty, is an aid to clear thinking.
[Note 2.]—Good usage sometimes permits a comma to be used before a conjunctive adverb in short sentences where the break in the thought is not formal or emphatic. For instance, when the conjunctive adverb so is used as a formal or emphatic connective, a semicolon is desirable (I won't go; so that's settled). But in the sentence, "I was excited, so I missed the target", a comma is sufficient. For the use of so is here informal, and probably expresses degree as well as result. (Compare "I was so excited that I missed the target").