The principle of basso continuo, or figured bass, is simple: We specify with figures the intervals we want to add above a given bass. The notes maintain the key's tonal alterations so that the same chord symbols can apply to the different types of triads. For example, a triad in root position was indicated with the numbers 5 (fifth) and 3 (third):
Nevertheless, to avoid using too many symbols, which would make reading the music harder and would complicate the process of copying the music, we assume that a chord is a triad in root position if there isn’t any symbol written below it. In the next example, all the basses without symbols should be harmonized with triads in root position:
Some schools use the symbol for the VII degree chord to indicate that it is a chord with a diminished fifth.
In first inversion, we use 6 and 3. But usually, it is simplified by just using the 6 alone. For this reason, we know these chords as sixth chords:
In second inversion we use 6 and 4 and don't simplify the symbol. These chords are known as six-four chords.