Semitone
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A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music, and it is considered the most dissonant when sounded harmonically. Wikipedia
Ratio: about 1.059 to 1
Just interval: 16:15, 17:16, 27:25, 135:128, 25:24, 256:243
Just intonation: 112, 105, 133, 92, 71, 90