In mathematics, an orered pair is a collection of two distinguishable objects, one of which is identified as the first coordinate (or the first entry or left projection) and the other as the second coordinate (second entry, right projection). The usual notation for an ordered pair is (a, b), with first coordinate a and second coordinate b. (This notation could be confused with that of an open interval on the real number line; the variant can be used to remove this ambiguity.) The pair is "ordered" in the sense that (a, b) is different from (b, a), unless a and b are the same.