X-N. Xristus Nica, on copper coins of Constantine V and Leo IV, Leo IV and Constantine VI, Constantine VI and Irene. Sometimes the letters are triplicated as on coins of Irene:
X-N
X-N
X-N
Sabatier, Mon. Byz., and Madden, Christian Emblems on Coins of Constantine I, etc, in Num. Chron. N.S., 1878, vol |xviii| p 175.