The job of a Piano Tuner is first, to get the piano into tune and second, to get it to stay there, more or less, so that subsequent tuning can be both more accurate and less time-consuming. This is achieved by the correct manipulation of the string, via the tuning pin, so that the string, when struck, doesn't deviate from the pitch at which it's been set. Also, as I have said elsewhere, a very out of tune piano will be difficult to render stable, due to the increase in tension throughout the instrument and will therefore require more tuning.

It makes sense, therefore, to keep your piano in tune by having it tuned regularly, at least once a year, if stable, but, preferably, twice, or more. In the interim, I’m happy to come and correct the odd out of tune note. Most pianos can achieve some sort of stability as long as they are kept in a suitable environment; see the section on this site “Your Piano”. 

Pianos that haven't been tuned for years can never be left stable on one tuning, especially if they are older instruments. This kind of piano, if very out of tune (and I come across plenty of them) will probably need to be brought to, at least, a standard pitch throughout, then given a further tuning and, preferably, another tuning within a month, at most.

Real tuning involves getting the instrument to sound sweetly throughout all its registers and once a piano is stable ideally involves moving the strings very little, or not at all, if not necessary. Crucially, the unisons, that is, the three or two strings comprising, respectively, the trichords and bichords of each note must be tuned absolutely beatless if the chords that contain them are to sound in tune. Additionally, the top treble section, which is the hardest area of the piano to tune, should be tuned sharp enough for its notes to ring sympathetically with those both above and below which, if achieved, can drastically enhance the overall tone of the piano. Here, though, we encounter a problem known as “false beating”, where one, or two, or indeed all of the strings of a trichord each have their own beat, sounding like two strings out of tune with each other. This makes a note untuneable for the purposes of the chord. It is a fault that exists in most pianos to some degree, heard most keenly in the upper registers and the tuner can only tune these notes as best they can.