What's the difference between a key and a scale?

a scale is a finger exercise running up and down the notes of a specific key.

People speak of non-Western scales. Are there non-Western keys? Does that even make sense as a question?

The reason you don't use "key" and "scale" interchangeably is because the key of a piece usually features more notes than just those contained in its corresponding "diatonic" major scale. Furthermore, scales cannot actually produce harmony on their own - that takes more than one voice, using notes from a scale.

Think of the key as the "big picture" harmonic structure of a piece, and a scale as a building block of that structure.

hmm, its a bit of a chicken/egg situation, they are both governed by the same harmonic rules

People speak of non-Western scales. Are there non-Western keys? Does that even make sense as a question?

er, sort of. keys do tend to be a western thing though, most non-western instruments (and some western ones) are designed to be played in only one key. eg. the bagpipes & sitar are defined by the drone that accompanies them, so are grounded in the same key.

a key is the harmonic context for the piece. it sets two things: the root, or natural harmonic 'home base', and the set of 'legal' notes, although there are plenty of ways to include other notes.

a scale is an ordering of notes that sets a melodic context. all scales have a key, i.e. a harmonic context, and share the root or 'home base' with that key. however, scales are selections of notes from their key that provide the wherewithal to create melodies which have certain tonalities. so, a melody created from a pentatonic scale in D will have a different flavor from a melody created from a Dorian mode scale in D (that one, assuming a particular rhythmic approach, will sound mostly like british isles folk music). they both will share D as a root note but will visit different places along the way (and will have different numbers of available notes).

further complicating the matter is that many scales have the exact same notes but in different orders with different roots, and have different names. A C major scale has the same notes as a D dorian mode scale (i think), but they have very different tonalities because of the places they start and end. that is one of the most interesting things about musical tonality - it's not in the notes, it's in the sequence.

The key describes the harmony; the scale the melody.

Is that way off, do you think (apart from it necessarily being very much over-simplified)?

This is true, except that when you talk about modes, you're no longer talking about tonal harmony. Church modes were designed to facilitate *counterpoint* (as opposed to "harmony"), and at that time, there wasn't a tonal harmony to speak of. In fact, our notions of harmony come out of the modal counterpoint.

I think for someone getting into music theory today, what is really hard is that very little popular music actually follows the "rules" of harmony, as you might learn in college. Just on a basic level, you don't hear as many V to I cadences in pop music. You hear lots of IV to I, and other variations. This is kind of an interesting situation, because even dating back to the use of church modes, and going through all periods of classical music up until the 20th century, emphasizing the "leading tone" just before returning to the root was the backbone of Western harmony. (And this doesn't even take into account atonal or other non-traditional harmonic structures in *most* experimental and modern classical music.)