Root & 5th
Music's common denominator
The root, octave, and perfect 5th are the fundamental building blocks of music as you know it.
They underlie seeing your instrument as a stack of octaves, with sets of notes repeating in each octave relative to a stable root — the most common pattern in modern music. They are the first and most consonant steps of the harmonic series, and so are fundamental to sound itself. Unlike the octave, which connects notes of the same pitch class, the 5th is the keystone of musical movement, embodied by the circle of 5ths.
Most practically, they are contained by all the most common, useful notesets — the major and minor triads, tetrads, pentatonics, and every popular mode — which makes them central to NoSheet's progression. They are the innermost layer of the nesting structure.