One of the main purposes of design right protection is to protect more functional designs. There is, however, an exclusion for those aspects of the design which ‘must fit’ with other articles in order that either article may perform its function. It is only the interface that is excluded from protection i.e. the bits that have to fit into up or around the other object. The focus will be on whether the designer truly has any freedom of design open to him. If there is some freedom to vary the design, the exclusion will not offer a defence to a copier of the design. If the spare part designer does have latitude in designing spare parts he would be expected to use or face infringement claims.
This exclusion also applies to human spare parts (prostheses), or contact lenses. A further example of this is for holders for mobile phones, many features of which may not qualify for an unregistered design right as its shape may be dictated by the phone.