Maoz Tzur is a 12th or 13th century poem, sung immediately after lighting the Chanukah candles. Exactly who the author was is unclear, though the first letters of each verse spell the name Mordechai. There are several melodies, but this one is by far the most popular. It is certainly of non Jewish origin: a similar tune was used by Martin Luther for the 16th century hymn "Now rejoice dear Christians". Some have suggested that both melodies were based on the earlier German folk song "So weiss ich eins, dass mich erfreut, das pluemlein auff preiter heyde". Before being used for Maoz Tzur, this melody was sung to another Chanukah poem, Sh'nei Zeitim which is said by some on Shabbat Chanukah. In the 19th century, Julius Mombach modified the second line of the melody, so that it modulates to the dominant, adding impetus to the melody through tension and resolution. This variant was included in the Blue Book as traditional without Mombach's name attached, and spread to many UK congregations. In the UK this is typically how the melody is still sung today; in other places the second line is a direct repeat of the first line. Other variants do exist e.g. the Frankfurt melody has a different final line to each verse. This arrangement follows Mombach's altered melody. Like Adir Hu and Akdamut, the Maoz Tzur tune is used as a seasonal leitmotif in many communities for Mi Chamocha in Ma'ariv, Hodu and Ana in Hallel, Shir Hama'alot, Adon Olam and Yechadshehu for Rosh Chodesh (Kislev and Tevet).