Pay attention to the choruses of the canon - one to the first and the other to the second troparion; these are verses from the 50th penitential psalm (which is read immediately before the canon itself). On the 9th song of the canon there is one more troparion; only a few prayer books indicate the chorus to this additional troparion - the next verse of the same psalm (the chorus is indicated in the Service Book). The choruses surprisingly enrich and complement the meaning of the troparia.