The siddur is the prayerbook used by Jews the world over, containing a set order of daily prayers. However, there are many additional liturgical variations and additions to the siddur for the Yamim Noraim ("The Days of Awe"; High Holy Days, i.e. Rosh HaShanah and Yom Kippur). As such, a special siddur has developed for just this period, known as a machzor (also: mahzor). A machzor contains not only the basic liturgy, but also many piyutim, Hebrew liturgical poems.
The Yom Kippur service opens in the evening with the Kol Nidre. Its name is taken from the opening words, meaning "All vows". A detailed article on the Kol Nidre provides more informaton.
The prayers devotions during the day are nearly continuous from morning until evening. Much prominence is given to the liturgical pieces in which the Temple ceremonial is recounted. This is known as the Avodah service.
Ibn Gabirol's ("Crown of Royalty") skilfully deals with the problem of sin: it is appended to the Sephardic liturgy for the evening service, and is also read by the more devout in the Ashkenazic synagogues.
In the center of the liturgy is the communal confession of sins. "For we are not so bold of face and stiff-necked as to say to You, We are righteous and have not sinned; but, of a truth, we are sinners. . . . May it be Your will that I sin no more; be pleased to purge away my past sins, according to Your great mercy, only not through severe chastisements."
The traditional melodies with their plaintive tones endeavor to give expression alike to the individual's awe before the uncertainties of fate and to a people's moan for its departed glories.
See also: Yom Kippur, Jewish services, Siddur