In response to Joyce's comments regarding the stability of tunings,
let me add that the technique of the tuner has a lot to do with it.
Having started tuning in the mid-sixties I've had some experience with
pianos of many sorts.
In some cases I've been called in to retune a piano that had been tuned
by another 'tuner' only a day or two earlier. I'm sure that the other
tuner was satisfied when they left the instrument but their tuning was
After doing considerable investigating into the problem I came across
a technique of tuning whereby I manipulate the tuning lever with the
left hand rather than the right hand which seems to be the 'norm' and
when I adopted this technique I was pleasantly surprised to discover
that it indeed made for a more stable tuning.
In summation, the tuner's method of tuning can greatly affect the
stability of the tuning job.