Some desktop publishing systems permit developers to create plug-ins (extensions that perform a variety of functions that the system itself lacks), and this capability has also been used for kerning. In general, these plug-ins permit the user to apply a kerning change automatically to a certain character pair throughout an entire document, instead of applying it by searching manually for those pairs. So far only basic kerning features have been implemented through plug-ins, and it is unclear whether the more advanced features can be effectively implemented in this manner.