  An algebraic structure related to discrete zero curvature equations is
established. It is used to give an approach for generating master symmetries of
first degree for systems of discrete evolution equations and an answer to why
there exist such master symmetries. The key of the theory is to generate
nonisospectral flows $(\lambda_t=\lambda ^l, l\ge0)$ from the discrete spectral
problem associated with a given system of discrete evolution equations. Three
examples are given.
