omitted words: when to use a comma
The comma that marks words left out of a clause because the preceding clause already supplies those words.
In an elliptical construction, the second of two clauses leaves out words that the first clause already supplies. When a semicolon joins the two clauses, a comma marks the place of the omitted words: The first draft took a week; the second, a month. The semicolon separates the two clauses, and the comma marks the omission within the second clause.
When a comma and a coordinating conjunction join the two clauses of an elliptical construction, no comma marks the omission: The first draft took a week, and the second a month.
When an omission leaves the second clause unclear, write it in full. In The first plan would cut the cost of maintenance; the second, of training, the second clause omits would cut the cost and may be unclear. Write The first plan would cut the cost of maintenance; the second would cut the cost of training.
See also semicolon between independent clauses; comma splice / run-on sentence; comma.
Examples
UseLena studied French; her brother, Italian. (In an elliptical construction joined by a semicolon, a comma marks the omitted verb.)
UseSome guests arrived by train; others, by bus. (In an elliptical construction joined by a semicolon, a comma marks the omitted words.)
UseMaria ordered the salmon, and Ben the steak. (In an elliptical construction joined by a comma and a coordinating conjunction, no comma marks the omitted verb.)
AvoidLena studied French, her brother, Italian. (A comma alone cannot join two independent clauses, and the second comma then reads as part of a list. Write Lena studied French; her brother, Italian, with a semicolon after French.)