Sometimes we hear musicians say things like "That's a modal tune" or "That's in a minor key, but one note's different," etc. Let's talk about what that means!
In the Middle Ages and earlier, music was essentially just melody and rhythm, with no harmony, and besides that the instruments could play in only one key. (I find the no-harmony thing hard to imagine, but that's what music historians say.) To get variety and color in the music, then, one thing musicians did was to set up a system of "modes" using that single-key scale. And the mode system is used in most music today as well. This is actually very easy to understand, and it's a helpful thing to know if you play music yourself!
Nowadays the majority of music is played in four common modes, two of which we all probably know as the major and minor scales. So half of the modes are ones we already know! Here's how they're built:
The "do-re-mi-fa-sol-la-ti-do" scale is also known as 1-2-3-4-5-6-7-1, and the pitch difference between 3-4 and 7-1 is half as big as the others (it's a "half step"). That's the Major Scale, and it was known in olden times as the Ionian Mode. Same thing, different name. (In the key of C, the notes are C-D-E-F-G-A-B-C.)
The natural Minor Scale uses exactly the same seven notes as the major scale, but it starts on a different pitch in the series: It goes 6-7-1-2-3-4-5-6, and because of the resulting shift in where the half-step pairs are, it has that famous minor sound. In early music it was called the Aeolian Mode. (In the key of C it’s A-B-C-D-E-F-G-A, or “A Minor.”)
Now let’s look at the other two:
The Mixolydian Mode also uses the exact notes of the Major Scale, but it follows the pattern 5-6-7-1-2-3-4-5; the half-step pairs again have shifted, so that this mode sounds like a Major Scale with the seventh pitch flatted (“4” in the series). It’s common in many genres, such as Old-Time (“Old Joe Clark” is a good example, with the flatted seventh step on the third note of the melody), Bluegrass, Rock, Classical, Blues, etc., etc. (In the key of C it’s G-A-B-C-D-E-F-G, or “G Mixolydian.”)
The fourth is the Dorian Mode, again using exactly the same notes but with the pattern 2-3-4-5-6-7-1-2, and the shifted half steps render what amounts to a minor scale with a sharped sixth pitch in the climb (“7” in the series). There’s an intriguing sense of mystery or contemplation, perhaps, in the mood of this key: You can hear it in “Scarborough Fair” on the last syllable of the word “rosemary.” (In the key of C this mode is D-E-F-G-A-B-C-D, or “D Dorian.”)
To get more familiar with how these are set up, I suggest that you go to your instrument and sound out an octave 1 through 1 (say, C to C), 6 through 6 (say, A to A), 5 through 5 (say, G to G), and 2 through 2 (say, D to D), listening for the sounds and seeing how the patterns look. Then simply keep in mind that these modes exist as you run across the characteristic sounds in the music you play or compose.
It makes so much sense! Let’s demystify our ideas of music a bit!
Summary in chart form:
...And I've made a short video showing this on the hammered dulcimer, too, here: