By Nicholas D. Lewis
This post will be relatively short, but I feel that this topic needs to be discussed. Two musicological gadflies at my old high school in Pennsylvania were debating about the difference between a scale and a mode as I sat silently petrified in the corner. I listened to their conversation and heard reasonable arguments on both sides, but they were off the mark nonetheless. In this article I am going to briefly address the difference. It turns out that most people have a theoretical misunderstanding of this topic, so I hope those people will take this presentation to heart.
In modern music theory, a scale is actually a device for measuring the distance between oitch classes. Abstrically, a scale simply defines an intervalic “scale step”. From this point of view, they are like a geometric ruler that define intervals by their relation to other pitch classes in scale. This definition leads to some surprising results, and this is where most people get things wrong. Scales do not have starting notes (also called “tonics”). What is commonly called an “A minor scale” could actually also be a “C major scale”.
Modes, on the other hand, are defined by their starting pitch-class. A diatonic scale with no sharps or flats that starts on the pitch class “D” is in Dorian Mode. When the starting note of dorian mode is D, it is also a C major scale or A minor scale. A B altered jazz scale is in super locrian mode.
The bottom line is that scales do not have starting notes, but modes do have starting notes. Modes have tonics, scales do not have tonics. Scales define a distance (called “scalar distance”), but have little to nothing to do with tonality in general.