Introduction

The Orgelbüchlein consists of 46 organ chorales organized sequentially by seasons according to the church calendar. The following bullet list summarizes Bach’s large-scale organization:

Advent – Four chorales, BWV 599-602, are grouped into two duration pairs.
Christmas – Ten chorales, BWV 603-612, are grouped into two duration pairs and two groups of three.
New Year – Three chorales, BWV 613-615, are grouped into one duration group of three.
Epiphany – Two chorales, BWV 616-617, are grouped into one duration pair.
Lent – Seven chorales, BWV 618-624, are grouped into four duration pairs. The last chorale in the Lent section, BWV 624, is paired with the first chorale of the Easter section, BWV 625.
Easter – Six chorales, BWV 625-630, are grouped into one duration pair and one group of three, except the first chorale, BWV 625, which is grouped with the last Lent chorale, BWV 624.
Pentecost – Four chorales, BWV 631-634, are grouped into two duration pairs. The last chorale, BWV 634, is a duplicate of the previous chorale, BWV 633, the only difference a minor change in ornamentation. Thus, the two duration pairs here consist of BWV 631-632 and BWV 632-633.
Catechism – Four chorales, BWV 635-638, are grouped into two duration pairs.
Miscellaneous – Six chorales, BWV 639-644, are grouped into three duration pairs.