The vec operator of a matrix X consists of stacking columns of X one under another in a single vector. The vech operator of a square matrix X is obtained in the same way that vecX is defined, but it starts each column at its diagonal element. The objec-tive of this paper is to describe vec and vech operators and apply them to a multivariate mixed sire model.

The vec operator of a matrix X consists of stacking columns of X one under another in a single vector. The vech operator of a square matrix X is obtained in the same way that vecX is defined, but it starts each column at its diagonal element. The objec-tive of this paper is to describe vec and vech operators and apply them to a multivariate mixed sire model.