Operation of grain boundary sliding is examined for conditions of plastic strain incompatibility that is the most frequent case for deformation of polycrystals. Two coexisting components of grain boundary sliding: dependent and independent on intragranular slip are distinguished. Theoretical estimate of a ratio between slip induced sliding and intragranular slip is obtained. It is concluded that at the beginning of deformation intragranular slip and grain boundary sliding behave independently. However, at high strains they can be viewed to some extent as interdependent processes.