Switching-valve assembly for switching between two respiratory gas sources (88a, 88b, 88c) of a respiratory device, where the switching-valve assembly comprises: - A housing with a first source-linking aperture, with a second source-linking aperture arranged spatially at a distance from the first one and with an output-linking aperture arranged spatially at a distance from the first and from the second source-linking aperture (26, 54), - A first gas supply line which connects the first source-linking aperture with the output-linking aperture, - A second gas supply line which connects the second source-linking aperture with the output-linking aperture, - A valve arrangement (40, 42, 74, 76) accommodated in the housing which is displaceable between a first operational state in which it releases the first gas supply line for flow-through of respiratory gas and blocks the second gas supply line and a second operational state in which it blocks the first gas supply line and releases the second gas supply line for flow-through of respiratory gas, Characterized in that the switching-valve assembly is configured stackably as a modular switching-valve assembly, in that the housing exhibits a first outside section with a first stack exterior surface section and a second outside section arranged at a distance and/or at an angular distance relative to the first outside section with a second stack exterior surface section configured complementarily to the first stack exterior surface section, where the output-linking aperture is configured at an outside section out of the first and the second outside section (22, 14) and where a source-linking aperture out of the first and the second source-linking aperture (26, 54) is configured as a follow-up linking aperture at the respective other outside section out of the first and the second outside section (14, 22), namely in such a manner that when the first and the second stack exterior surface sections (16, 24) are conceived as stack exterior surface sections (16, 24) of similar separate modular switching-valve assemblies in stack engagement with one another, gas supply line sections (20, 28) each extending from the outlet- and the follow-up linking apertures (18, 26) into the housing are connected into a continuous gas supply line region. - A housing with a first source-linking aperture, with a second source-linking aperture arranged spatially at a distance from the first one and with an output-linking aperture arranged spatially at a distance from the first and from the second source-linking aperture (26, 54), - A first gas supply line which connects the first source-linking aperture with the output-linking aperture, - A second gas supply line which connects the second source-linking aperture with the output-linking aperture, - A valve arrangement (40, 42, 74, 76) accommodated in the housing which is displaceable between a first operational state in which it releases the first gas supply line for flow-through of respiratory gas and blocks the second gas supply line and a second operational state in which it blocks the first gas supply line and releases the second gas supply line for flow-through of respiratory gas,