Pseudepigraphal letters are not, of course, distinguishable in form from authentic letters, of which they are fictional imitations. The diagram does not include type CP, which could in theory exist, but would be a rather sophisticated fiction, involving two levels of fiction, and is therefore unlikely. In fact, as will become clear, type AP6 could well be regarded in another sense as the pseudepigraphal equivalent of type C. All letters, including pseudepigraphal letters, must specify both sender(s)