The organ is unique amongst musical instruments in that it makes an architectural contribution to the building in which it stands. This is not solely on account of size: the layout and decoration of organ cases has traditionally been a branch of architectural design with its own grammar and traditions, but still allowing free interpretation according to changing fashions, local influences, and the skills of the designer and organ builder.

The organ case has an effect on the sound of the organ, though this may be difficult to define. For the neo-classical builders of the mid-twentieth century the revival of the traditional organ case – with side walls, back and roof – was an important argument in the ideology of organ reform. The casework was believed to focus the sound, assist the blend of the various ranks of pipes and project the sound into the room (usually down the main axis of the building). There is still considerable debate as to whether the organ case is essentially a passive structure, like a loudspeaker cabinet, or whether it has an active role as a resonator, bringing it more into line with other musical instruments where the body or soundboard is vital in creating power and timbre.

In practice many variations are possible, and as with other factors in the design and building of organs, success is not dependent on this element alone. If one allows that good organs exist in many forms and from many different periods, then one must acknowledge the success, on their own terms, of organs built in less than ideal cases or indeed without any case at all.