The Service of the Word includes the readings, responses to the readings, the sermon, and the Prayer of the Church. The readings consist of the Lesson (usually from the Old Testament), the Epistle (from one of the New Testament epistles), and the Gospel (from one of the four gospels).
In general, the lessons for the day are chosen according to their thematic relationship. The list of selected lessons is called a pericope or lectionary. For many years all the liturgical churches used the same list, called the Historic Lectionary. This list was compiled in the ninth century and was used almost exclusively up until the mid 20th Century. This made it possible for pastors to study texts together because they were using the same ones. Pastors can also study what other preachers, particularly Martin Luther, have written on the texts.
Starting a few decades ago, a new lectionary, called the Common Lectionary was developed. Instead of covering the same texts in one year, it covered more of the Bible over three years. While most Lutheran churches now use a version of the three-year lectionary, a significant number use the Historic Lectionary or alternate between the two.
At St. Timothy we commonly alternate between the Historic and the three-year lectionaries as they appear in The Lutheran Hymnary. In addition, the propers (the changing parts of the service) are taken from The Lutheran Hymnary as well as the tones for chanting the service.
In addition to the two lectionaries mentioned above, there are a number of minor lists. Since all of them follow the church year, they are thematically similar and use the same propers (collects, graduals, etc.).