A hint as to the nature of these curious compositions will be sufficient, perhaps, to mystify the uninitiated reader. The mass, _Ad omnem tonum_, shows, instead of the clefs, question marks as signatures; and it may be sung, by using the corresponding accidentals, in any church mode. The thirty-six part mass, with canon for nine parts, already mentioned, is not a 'riddle,’ but has all the difficulties of one. In Okeghem’s school is found the so-called 'crab canon,’ _canon cancrizans_, which is first sung through in the usual way from beginning to end, then repeated backward. There is also a canon which, like the _canon cancrizans_, is to be sung through twice, but from the beginning to the end both times. In the second singing, however, each progression of the original melody _down_ is answered by a corresponding interval _up_, or _vice versa_. This is known as the 'inverted canon.’ One of Okeghem’s followers, Hobrecht, furnishes us even with a canon which has both the retrograde and the inverted motion. In fact, canonic forms of all varieties and complications were treated by Okeghem and his school to the point of exhaustion. It must not be forgotten that the range given to a single voice was much more limited than at present; that these compositions must conform to the strictest rules, not only when sung in the normal manner, but when repeated in retrograde or inverted motion; and that the very essence of the work was the perfection attained in adhering to contrapuntal laws, rather than the expression of individual feeling. Okeghem himself made these puzzles but rarely, and, as it were, in the manner of providing an intellectual treat for the educated musicians of his day, especially those who formed the church choirs. These difficult works were a test of their ability and thorough acquaintance with Church modes; they afforded exercise in transposition from one mode to another and offered the charm of variety which the special characteristics of each individual mode imparted. Furthermore, they tended to develop the highest artistry the vocalist was capable of, and were an illustration of the variety of combinations possible with the already existing parts.