The tuning curve is the calculation of offsets for each note to compensate for inharmonicity. You can view the tuning curve from a function on the main tuning screen. The use of 3-part tuning (as described here) is the preferred pattern as since it is responsive to inharmonicity on a note by note basis. The amount of stretch can be customized for personal preference through the selection of intervals for the bass, mid-range, and treble. The older template for tuning curve calculations (before 2019) is not recommended, but it is still available for backward compatibility with saved tunings.
When creating custom tuning, TuneLab listens to a few notes to measure
their inharmonicity. You can use as many or as few notes as you
like, so long as at least four notes are measured. Each measurement
automatically registers as many partials as TuneLab can hear and all of
them are used to calculate the inharmonicity constant for the note.
Repeated measurements of the same note are automatically averaged by
TuneLab. Then TuneLab takes whatever inharmonicities were measured
and calculates the inharmonicity for all 88 notes - even for the notes
that TuneLab did not listen to. Of course, the more notes that you
let TuneLab listen to, the better this calculation will match the actual
pattern of inharmonicities in the piano.
The curve shown just below the tuning curve is a graph of how wide or
narrow certain selected intervals will be, based on the current
inharmonicity measurements and tuning curve calculation. It is not
necessary to refer to the Deviation Curve unless you are experimenting
with different tuning curve settings and you wish to evaluate them without
actually doing the tuning by seeing how certain selected intervals are
projected to sound. The bass and the treble section of the deviation
curve may be set up to display from a long list of intervals. You
may select different intervals for the bass and the treble. Select
the intervals that are important to your tuning and see exactly how far
each interval is from being beatless.
The list of tuning curve offsets is just one component in the total tuning file. There is also an optional table of custom stretch offsets that you can alter separately for each note. These apply above and beyond the calculated offsets and are rarely used. If you have applied an historical temperament, it becomes another component of the total tuning. These components are maintained separately in the tuning file so that they can be independently adjusted or removed at some later time if desired. Finally, there is an optional comment or notation that is arbitrary text that you can associate with the tuning. This can be used to store comments about the tuning, such as repairs that may be needed in the future, or what to expect the next time you are tuning this piano