Realization
Writing out the figured bass.
What it means
Realization is the term for the execution of a figured bass: producing, from the bass note and the numbers standing beneath it, the actually sounding texture. The figuring indicates only which intervals are to sound above the bass; position, number of voices, voice leading, and figuration are left to the player. Realization is therefore not the decoding of an unambiguous prescription but a shaping task.
In the notation
In practical editions the realization often appears written out in the upper staff, usually in smaller print, so that it remains recognizable as coming from the editor and not from the composer. Urtext editions dispense with this or offer it separately. The numbers themselves count the intervals above the bass: no figure means the triad in root position, 6 the six-three chord (first inversion), 6/4 the six-four chord, 7 the seventh chord with its inversions 6/5, 4/3, and 2.
Musical use
The figured bass was, from the early seventeenth to the late eighteenth century, the foundation of nearly all ensemble music, and the ability to play from the figuring belonged to the basic training of every musician. Execution was guided by the circumstances: thinner with accompanying recitatives, fuller in festive movements, different on the harpsichord than on the organ or theorbo. A good continuo player supports the singers without covering them, and fills in without disturbing the voice leading of the upper voices.
It differs from an arrangement in that a realization adds nothing foreign to the work but executes what the notated text leaves open.
A good realization is guided by instrument, scoring, and style: a harpsichord demands a different texture than an organ or lute, and a recitative different chords than an aria.
Performance practice
Begin in three voices in close position and with movements as small as possible between the chords; this yields almost automatically a clean voice leading. Do not grip more notes than the texture can bear, and leave room for the upper voice — the continuo part should not lie above the melody. Read the figures as distance from the bass, not as a fingering shape. And listen to the singers or solo instruments: what happens there determines how much you play.