The church modes, or “Ecclesiastical Modes,” are the ancient predecessors of the major and minor scales. Each of the modes is a rotation of the major scale. That is, if you take any major scale and start on a different note, you get a different mode. The modes are named the same way as a scale: the root of the mode with the type of mode. With modes, the key note is called the final instead of the root. The basic modes are known as the “authentic modes.”
The major scale is also known as the Ionian mode. Using the white keys on a piano, this mode goes from C to C. Thus, the C Ionian mode is: C, D, E, F, G, A, B, C.
Still staying on white keys, the first rotation that begins and ends on D is called the Dorian mode. The next rotation, beginning and ending on E, is the Phrygian mode. Next comes the Lydian mode, based on F, and then the Mixolydian mode from G to G. The sixth mode is called the Aeolian mode, from A to A, with the Locrian mode from B to B rounding out the set.
The above modes may be transposed to any key. You can have a C# Phrygian mode just as easily as an E Phrygian mode. It can get hairy thinking of rotating through the series of intervals when transposing modes; how many half steps, and how many whole steps, and….? It’s an ugly business. For that reason, I specifically left out the sequences of intervals in the descriptions above. The much easier way is to figure out what scale – major scale, that is – the mode is in, and then rotate through it until you get the mode you want.
Say, for example, you want to write out the C# Phrygian mode. The Phrygian mode starts on the third scale degree of the major scale. In C major, that’s E. What major key has C# as the third scale degree? If it doesn’t come immediately to mind, just count backward: the second scale degree would be a whole step lower, B, and the root would be another whole step below that, A. The C# Phrygian mode, then, is the third mode on the A major scale.
Additional modes, called plagal modes, are constructed with the prefix “hypo-”, meaning “extending below.” These modes, instead of extending over an octave from final to final, extend a perfect fourth below the final and go a fifth above it. You still have an octave, but now the final is in the middle instead of at the ends. Speaking strictly in terms of pitch material, these plagal modes are identical to authentic modes. You have the same notes in the D Dorian mode as in the G Hypomixolydian: D, E, F, G, A, B, C, D. The difference is that in the D Dorian mode, D is the final, whereas in the G Hypomixolydian mode, G is the final. This difference has to do with usage in 16th century counterpoint, which is beyond the scope of this description. For modern intents and purposes, the seven main modes should be sufficient.
The difference in ordering of the intervals of each mode gives them distinctly different characters. This difference is usually characterized as “bright” versus “dark.” You can think of brighter modes as pushing upward, away from the bottom final, as the darker modes pull downward toward it. Once again sticking to the white-key modes (rotations of the C major scale), modes get darker as the final progresses in the circle of fifths and brighter as the final moves backward in the circle of fifths. That is, the brightest of the white-key modes is the F Lydian mode, and the darkest is the B Locrian mode. From darkest to brightest, the modes are: Locrian, Phrygian, Aeolian, Dorian, Mixolydian, Ionian, and Lydian.
Note that if you take those seven modes and set them on the same final, exactly one note moves upward or downward by half a step as the mode gets brighter or darker, respectively.