Ellipsis in quotations
Also called: ..., omission marks
Three spaced or unspaced dots showing that you have cut words out of the middle of a quotation.
An ellipsis marks an omission. It goes where the cut is, and its whole purpose is to keep you honest: the reader can see that material is missing and that you are not presenting a stitched-together sentence as continuous prose.
You do not need one at the start or end of a quotation — a reader already assumes the sentence came from a longer text. You do need one for a mid-quotation cut, and you must not use one to remove a qualifier that changes the meaning. Cutting “rarely” out of “rarely effective” is misquotation even with the dots in place.
Styles differ on the typography: APA uses three spaced periods, Chicago allows both, and a cut spanning a full sentence often takes four points (a period plus the ellipsis).