Abstract: This invention relates to modular building construction apparatus and in particular to a mold having movable framework used to construct a basic building module where any miss-alignment at the junction of framework elements is enough to make the imperfection noticeable. If the imperfection is a ridge it will be readily apparent on the inside surface of the formed building module and it will not be disguised by painting over it. What is disclosed is a movable corner arrangement for a mold for forming at least one of the inner corner surfaces of a moldable building module. The inner mold for a moldable building module has an upper support panel for forming the ceiling surface of the building module and a plurality of movable side walls that depend from the upper support panel using flexible peripheral edge elements. The side walls have mold facing side wall surfaces movable to a position to form corresponding internal wall surfaces of the moldable building module.