Piano strings are attached to a cast iron frame, also known as harp.
Cast iron is much less sensitive to humidity and temperature changes than wood.
This is the main reason why they stay in tune relatively well.
In contrast, harpsichords are somewhat similar but their frames are made of wood and they require much, much more tuning.
Other factors are that that pianos aren't usually moved and won't get knocked out of tune.
They usually require tuning after moving.
The metal strings are stable once initially settled, unlike synthetic or gut strings.
Playing the piano also doesn't involve touching the strings with your hands, which puts dirt and moisture on the strings.
music.stackexchange.com/users/56741/ojs
Most pianos can hold their tuning quite well, but can do so only under ideal conditions.
Because pianos weigh quite a bit they tend to not get moved around very much and are usually kept where temperature and humidity hold a fairly steady level over an extended period of time.
Expose a piano to the same conditions that smaller more portable instruments are subject too and you find they need to be tuned quite often.
Move a piano and it will probably need tuning.
Expose it to considerable temperature change, either hot or cold, and it will probably need to be tuned.
Although piano strings are considerably heavier in weight and gauge, they are still susceptable to the laws of physics just the same as violins and guitars.
music.stackexchange.com/users/44636/skinny peacock
They do, and they are!
But only really with concert grands prior to a concert.
Not as far out as maybe a guitar or violin might be, but just a subtle tweaking. Guitar and violin, for starters, have exposed tuners which get knocked easily - in and out of its case, for example.
Pianos are different.
The mass of a piano also means its tuning is more stable, but over time it will still go out of tune, humidity and temperature differences being the main culprits - which also affect guitars and violins more rapidly, and to a greater degree as well.
Stretching strings on guitars will affect tuning, and that's one thing we don't do on pianos!
music.stackexchange.com/users/2808/Tim
In critical concert and studio situations the piano WILL be tuned before each performance, and sometimes even 'touched-up' during it.
But the design and construction of a piano is such that they hold tuning pretty well, and in domestic situations tuning twice a year or even less frequently can be practically sufficient.
Other instruments can hold tuning too.
In Carol Kaye's instruction book - bible to a generation of bass guitar players - she recommends checking the tuning 'at least once a week'!
A friend, who used to stay with me when he visited London, died 10 years ago.
He left an acoustic guitar at my house.
When I opened the case recently, it was still perfectly in tune!
music.stackexchange.com/users/17003/Laurence Payne
It very much depends on strings and their fixtures.
Pianos have steel strings on a steel frame: short of moving the piano or changing temperatures, not a lot happens here.
On a violin, the E string usually is a steel string as well and mostly needs fine tuning.
Guitars similarly: steel strings need less tuning action than nylon strings, so concert guitar players tend to spend more time tuning than, say, Western guitar players (which is a tiny saving grace for 12-string guitars).
But wooden neck and corpus still move more under different temperature/humidity than a cast-iron frame.
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I usually tune my guitar and violin whenever I play them. I assume most people do too. But what makes piano strings different?  I know it is a lot of work to tune a piano, but I'm under the assumption that pianos do not need to be tuned as often as say a guitar.  Is this incorrect?
Why do pianos not need to be tuned every time you play?
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<piano><tuning><instruments>
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music.stackexchange.com/users/25276/Al Jebr
Piano strings are attached to a cast iron frame, also known as harp. Cast iron is much less sensitive to humidity and temperature changes than wood. This is the main reason why they stay in tune relatively well. Other factors are that that pianos aren't usually moved and won't get knocked out of tune. They usually require tuning after moving. The metal strings are stable once initially settled, unlike synthetic or gut strings. Playing the piano also doesn't involve touching the strings with your hands, which puts dirt and moisture on the strings. Most pianos can hold their tuning quite well, but can do so only under ideal conditions
Piano strings are attached to a cast iron frame, also known as harp. Cast iron is much less sensitive to humidity and temperature changes than wood. Most pianos can hold their tuning quite well, but can do so only under ideal conditions. Playing the piano also doesn't involve touching the strings with your hands, which puts dirt and moisture on the strings. The metal strings are stable once initially settled, unlike synthetic or gut strings.