Category:Works by musical mode
Traditionally there are 8 Gregorian modes. Each has an associated psalm tone and hence one sees designations such as primi toni (of the 1st tone) as well as primus modus. The tonus peregrinus, named for its changing reciting tone, is sometimes used with antiphons in mode 1.
Renaissance theorists came up with more complicated schemes of 12 modes or even more. These were eventually simplified to major and minor. The 7 white-note modes of modern theory are more recent still and borrow only their names from ancient Greece.
View the Wikipedia article on Gregorian modes.