Coxeter initially represented bifurcating diagrams with vertical positioning of numbers, but later abbreviated with an exponent notation, like [...,3 ] or [3 ], starting with [3 ] or [3,3 ] = or as D 4. Coxeter allowed for zeros as special cases to fit the A n family, like A 3 = [3,3,3,3] = [3 ] = [3 ] = [3 ] = [3 ], like = = .