Triad Inversions and Figured Bass

A triad is built by stacking thirds: root, third, fifth. When the root sits in the bass, that's root position, and it gets no figures or the figure 5 3, often left blank. When the third sits in the bass instead, that's first inversion, labeled with the figure 6, because the bass note is now a sixth below the root. When the fifth sits in the bass, that's second inversion, labeled 6 4, since the bass sits a fourth below one upper tone and a sixth below another. The figures always describe intervals counted up from whatever note is in the bass, not from the root.

Why does the feel change if the pitches don't? Because the bass note anchors what your ear treats as the harmonic floor. Root position sounds settled and stable. First inversion feels lighter, often passing. Second inversion feels unstable, almost suspended, which is why it often resolves quickly into something else. None of that comes from new notes; it comes from which note is lowest.

This sits right after you learned to build triads and name them with Roman numerals. Now the Roman numeral tells you the chord's identity and root relative to the key, while the figure riding alongside it tells you the inversion. A V chord in root position is just V. The same G major triad with D in the bass is still V, the root is still G, but you add a 6 to show the fifth is in the bass. Inversion and chord identity are two separate layers of information stacked on the same symbol.