Scale Finder

Because they are answers to two different questions. At three or more notes there is a chord, so the table is chord-scales: which scales fit that chord, ranked, with each sounding note shown as a degree of the scale. At two notes there is no chord yet, so it falls back to containment: scales that contain every note you played, ranked by fewest added tones. The line under the table always says which question is being answered.

Both are measured as degrees above the chord, not as instructions. Tensions are the scale degrees that sit outside the chord and colour it, the ninths, elevenths and thirteenths. Dissonances are the degrees that clash with a chord tone in that particular context, most often a natural fourth over a major third. The Tension column then grades the whole scale as consonant, moderate or high.

Because more than one genuinely fits, and which you want depends on the colour you are after rather than on correctness. Over a dominant seventh the Mixolydian mode is the plain reading, the altered scale and the half-whole diminished scale are the coloured ones, and all three are correct over the same four notes. The ranking orders them by how much colour they add, not by how likely they are.

Yes. Each row links to that scale or mode reference page, so the ranked list is a way into the reference rather than a dead end. Modes with no scale-type route of their own link to their mode page instead; Ionian and Aeolian link to the major and minor scale pages, which are the canonical ones.