This talk discusses a variation of the fibred semantics of D. Gabbay called plain fibring, introduced with the aim of combining logics given by matrix semantics. It is proved that any logic obtained by plain fibring is a conservative extension of the original logics. It is also proposed a simpler version of plain fibring of matrix logics called direct union. This technique is applied to the study of the class of fuzzy logics defined by $t$-norms.