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That is true, and the smaller soundboard is less liable to changes due to humidity.

Prout,
The middle portion of the piano scale is the region that moves the most regarding pitch. Shorter string scales, (low break-point), will change pitch more per change in elongation than a longer scale.

Often in smaller pianos the first plain wire notes above the wound strings are at a very low break point. So they are harder to tune, move pitch more with any humidity change, and if not tuned carefully, will slip out of tune more than a longer scale.

The unfortunate truth about piano tuning stability is that the part of the compass used most is what goes out of tune the easiest.

Thanks BDB and Ed. I always learn so much here.

Ed I watched your video but there is a word which you use a couple of times which I cannot quite hear: to quote " depending on wether or not you moved the ???? " it sounds like FLYT?
y In particular y,ou use it just before then end and it seems it is essential to the meaning of the explanation.

It depends upon the design and quality of the piano.

I can think of a few "short" uprights that are much more stable than the taller - even by the same maker using quality materials. It's the design that's makes the difference. One example is the old Baldwin Acrosonic vs the old Baldwin 6000, both of similar age, condition and environment. With respect to achieving and maintaining reasonable tuning stability, the "shorter" Acro wins hands down.

On the other hand, a cheaply made piano will not stabilize no matter what you do. Sadly, the examples of these are too many to recount here.

There are two basic definitions of stability as it applies to pianos. One is within the control of the tuner and concerns how well the tuning has been set in place to withstand the heaviest playing. This is independent of the size of the piano. Although I have known unisons that are very slightly put go back in tune by themselves, unisons that have failed need the attention of a tuner to correct them.

The other concerns how well the general structure of the piano withstands the many changes of atmosphere between tunings and is not within the control of the tuner.

Some pianos, well tuned, can stay in tune for years on end. Another piano of the same make and model may not fare so well, even in the same room. This ability to stay in tune is independant of the size of the piano. A piano can go out of tune in this manner and still have the unisons intact and come back into tune again later as though nothing had happened.

Thanks for the replies. It seems I won't be calling him again.

My performance was...ehh; I don't think the piano affected it. The unisons weren't too clean but since the piano was voiced darker, the faster-beating high partials aren't as audible.

Yes, the kind of stability I'm talking about is the ability to stay in tune under heavy playing.

Even the finest concert grands in the most ideal conditions will have some degree of tuning shift under heavy (abusive) playing conditions.

The tuning slippage you experienced could be because the piano was horribly out to begin with and there was insufficient time to stabilize it.

Many performance venues also can have issues with lights heating the strings too much when the lid is open. Stage doors being left open to facilitate moving in other instruments, props and A/V equipment. All these things can send the temperament and octaves adrift. Wild unisons are most probably tuner error. Although if time is short the top treble often gets short shrift so a wild unison may be present there already and not get attended to.

One of the most stable pianos I've ever found are the Baldwin Acrosonic spinets, one of the shortest pianos ever made.