What are scales ?
A group of notes within a key
the Scale of C major uses the notes C-D-E-F-G-A-B-C
the Scale of G major uses the notes G-A-B-C-D-E-F#-G
And if any of that is confusing then you should check out the theory lessons on this site
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The word "scale" literally means "ladder" or "climb" - it's from the Latin word scala. It can be used to refer to any set of two or more pitches that rise (climb) through a range of one octave. In terms of songwriting or improvisation, a scale is the set of notes you draw from to make melodies and chords - it's the box of crayons you have to color with.
Boxes of crayons come in different sizes. They can have all the colors, like the chromatic scale ("chroma" is Greek for color), or just a few, like the pentatonic scales - they have just five notes. You might also hear terms like "hexatonic" (6 note) "heptatonic" (7 note) or "octatonic" (8 note) scale - those are just any sets of 6, 7, or 8 notes within an octave. Oh, and the octave doesn't count; our major scales are heptatonic.
You'll hear other terms, like "major" scale, which tells you the interval pattern is WWHWWWH (or TTSTTTS if you perfer "tone" to "whole step") starting from the tonic, or first note of the scale. "Minor" scales are any scales that have a note a minor third above the tonic - there are a bunch of those, like the natural minor, harmonic minor, melodic minor, Gypsy minor, Dorian, etc.
"Diminished" scales have a symmetrical pattern of either WH or HW that repeats through the octave. "Whole tone" scales are made up of only whole steps.
You'll also hear people talk about "modes" - that's from the Latin word for "manner", as in "manner of playing" - it means you're taking a scale and using different notes as the tonic. But they're just scales too, because they're made up of notes that climb through an octave.
As musicians we tend to practice scales, because almost all pieces of music have parts that are scalar, sections where the melody follows the scale up or down for a bit. Being comfortable with the common scale fingerings helps us play music fluidly.
And in terms of improvisation the value of a scale isn't telling you what notes to play - it's a guide to what notes NOT to play: notes outside a scale that matches the key of a song need to be treated very, very carefully if you're going to use them.
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