Psalm tones are melodic formulae used for the singing of psalms.
The simplest,
tonus in directum, sang without antiphones - free composed chants that vary from
feast to feast, resemble the lesson and prayer tones, perhaps even simpler than the
latter, with only two inflections - flex and metrum. A special kind of tonus in
directum is sang around Easter, having an intonation at the beginning.
The regular psalm tones though are those with antiphons. Sung at the beginning and
end of the psalm, the antiphons show definite modal characterists, and in order to
be connected with them, the psalm tones needed to be organized under the system of
the eight modes.