Most of the Keykos key types known to the kernel have an 8 bit field known as the data byte. In general this information modifies the meaning of the key and is interpreted by what ever code defines the key's behavior. If the key is a start key then the data byte is delivered along with the invocation message to the code obeyed by the domain designated by the start key. For kernel objects the kernel interprets the data byte directly. The holder of a key has no direct access to its data byte. The data byte is the direct and primitive Keykos support for facets.