With the exception of the cross and triple products, the above formulae generalise to two dimensions and higher dimensions. For example, addition generalises to two dimensions as ( a 1 e 1 + a 2 e 2 ) + ( b 1 e 1 + b 2 e 2 ) = ( a 1 + b 1 ) e 1 + ( a 2 + b 2 ) e 2 , and in four dimensions as ( a 1 e 1 + a 2 e 2 + a 3 e 3 + a 4 e 4 ) + ( b 1 e 1 + b 2 e 2 + b 3 e 3 + b 4 e 4 ) = ( a 1 + b 1 ) e 1 + ( a 2 + b 2 ) e 2 + ( a 3 + b 3 ) e 3 + ( a 4 + b 4 ) e 4 .