According to the equal temperament tuning system, octaves are equally divided into twelve notes. A scale is a series of notes selected among these twelve notes.
Each of these notes is called a degree. Each degree has its own name but is also often designated by a Roman numeral:
Two scales are distinguished from one another by:
For example, seven different scales may be built with the seven natural notes as in the following examples:
Each of these scales has a characteristic order of whole steps and half-steps. The first one is called major scale, the second one Third Gregorian mode or Phrygian mode. Those names refer to the specific structure of each scale.
A scale may be built starting with any note using accidentals in order to maintain the order of whole steps and half-steps. For instance, in order to build a major scale on note D, F and C have to be altered:
Such a scale is called the major scale of D. It is major because of its structure and a scale of D because D is the note upon which it is formed.
There is an infinity of scales. Scales may also be created while composing. Composers such as Claude Debussy, Olivier Messiaen, and Bela Bartok, among others, have done so in the recent past.
See S > Scales