Scales & Keys.....What's the Difference?

Confused by the difference between a scale and a musical key?  The distinction can be confusing at first.  Maybe this will help clear up the confusion.  

A SCALE is about order.

Scales are sequences.  They go up.  They go down.  Tones are always in a specific sequence or order.  Scales NEVER go part way up, double back, then finish the sequence.  They ONLY go up or down in a specified order.  That order may allow jumps over tones in the diatonic sequence and the order does not have to be all 7 tones in the diatonic scale.  The pentatonic scale is like that. 

Scale = Order

A KEY is about population. 

Keys tell you which tones are used in a piece of music.  Keys DO NOT determine the scale used within the key.  As we've seen there can be thousands of scales that work within any specific key. 

The scale we select within a key determines the relationships among the tones of that key and thereby allows us to construct melody and its supporting harmony.

Key = population