An apparatus for generating a three-dimensional object is provided. The apparatus may include a housing having a surface defining a build receiver to receive differently-sized build modules or to receive a plurality of build modules. The build modules may each include a build chamber to receive a layer of build material from a build material distributor. The apparatus may include an agent distributor to selectively deliver a coalescing agent onto portions of the layer of build material to be received from the build material distributor such that when energy is applied to the layer the portions of the layer coalesce and solidify to form a slice of the three-dimensional object.