In Byzantine modal theory (octoechos), the word "Plagal" ("plagios") refers to the four lower-lying echoi, or modes (Powers 2001c). Thus Plagal first mode (also known as "tone 5" in the Russian naming system (Suchy-Pilalis 2007)) represents a somewhat more developed and widened in range version of the first mode. The plagal second mode ("tone 6" in the Russian system) has a similar relation to the second mode, and the plagal fourth mode—respectively to the fourth mode. Though there is no "plagal third mode," the mode that one would expect ("tone 7") is called the "grave tone