As THE name semicolon, half a colon, indicates, the semicolon comes historically after the colon; but in practice it is more important-at least, in the sense of being more popular. If anybody uses one more than the two simple points, period and comma, that additional point is usually the semicolon. By its very form (;) it betrays its dual nature: it is both period and comma. As it is half a colon, so is it also a modified period and a strengthened comma.
Stronger, more decisive than the comma, the semicolon is slightly weaker, slightly less decisive than the colon, and considerably weaker than the period; it is, however, both slightly stronger and notably more elegant than the dash. (For the relative values of the points, see Chapter 11.) The semicolon, in short, sets off one part of a sentence from another more decidedly and more distinctly than does the comma; unlike the period, it does not end a statement. Except in certain rather literary contexts, the semicolon separates clauses, seldom phrases, rarely mere single words; those clauses may be-and often are-principal clauses.
Having left the intricate yet safely navigable Mediterranean of the various uses of the comma, one comes out into the Atlantic, where, for a safe crossing, one needs more than the utilitarian comma and the unavoidable period. Except in literary or aesthetic or philosophic writing, by far the most important additional requisite is the semicolon; and even there it outweighs the colon and those two supernumeraries, parentheses and the dash. Suddenly, perhaps a shade apprehensively, one realizes that the use of the semicolon is not so simple as one had thought.
Semicolons may occur between clauses; between phrases or other word-groups; and even between single words or between single words and short word-groups.