Formalising Architectural styles has been claimed as making variations impossible. We demonstrate that in the contrary, formalising enables to vary a style and to control that all the variations are still compatible with the style. To do so, we formalise the Pipe and Filter style together with several variations. We then show how to use these variations to develop the specification of a convolution product. The different solutions developed are proved equivalent and compared. The formal specification language LOTOS will be used as Architectural Description Language. The comparisons and other validations will be performed using the LOTOS environment CADP.