In this paper we investigate the phase behaviour of a `simple' fluid confined to a slit of nanoscopic width s<sub>z</sub> by chemically decorated, plane-parallel substrates consisting of slabs of weakly and strongly adsorbing solid which alternate in the x -direction with period s<sub>x</sub> . In the y -direction the substrates, occupying the half-spaces - z - s <sub>z </sub> /2 and s <sub>z </sub> /2 z , are translationally invariant. On account of the interplay between confinement (i.e., s<sub>z</sub> ) and chemical decoration, three fluid phases are thermodynamically permissible, namely (inhomogeneous) gaslike and liquidlike phases and `bridge phases' consisting of high(er)-density fluid over the `strong' part which alternates in the x -direction with low(er)-density fluid over the `weak' part of the substrate. In the x -y plane the two are separated by an interface. Because of their lateral inhomogeneity, bridge phases can be exposed to a shear strain s_x (0 ½) by misaligning the substrates in the x -direction. Depending on the thermodynamic state of the confined fluid and details of the chemical decoration, shear-induced first-order phase transitions are feasible during which a bridge phase may be transformed into either a gaslike (evaporation) or a liquidlike phase (condensation). These phase transitions are studied by computing phase diagrams as functions of s_x for a mean-field lattice-gas model. The lattice-gas calculations are amended by grand canonical ensemble Monte Carlo simulations of a fluid confined between chemically decorated substrate surfaces. The combination of the two sets of data reveals that the lattice-gas model captures correctly key characteristics of shear-induced first-order phase transitions in this rather complex system despite its mean-field character.