“I have an old piano that hasn't been tuned in years”...
You have just inherited your grandmother's piano or you found a piano on-line but you aren't really sure how long ago it's been since it was tuned. Chances are it is going to need more than a simple tuning. Pianos that are left for years without service will invariably drop in pitch as the string tension slowly releases over years. You as the piano owner may have to decide whether it is worth the extra effort and possible expense to bring your piano back to life...
There are a few factors to consider with a neglected piano. The first concern for most people will be tuning but the lack of overall maintenance, wear and tear in the action and the aging process should also be taken into account when trying to establish a piano's viability.
Pianos are stringed instrument just like a guitars or violins, however they are far more complicated. The average piano will have somewhere around 230 strings and has a complicated mechanism for each of the individual 88 keys that transfer the the player's touch to the swing of hammers striking strings.
Pianos are designed to stay in tune far longer than other stringed instruments but given enough time without tuning, the tension on the strings will slacken and the pitch will fall. Eventually it may fall so far below pitch that it will require significantly more time to tune or may even need more than one tuning to bring it back to concert pitch.
That said, it is possible to just tune a piano so that it is in tune with itself if the owner doesn't want to pay for the extra time involved in bringing their piano back to concert pitch, but that is a compromise. A piano that is below pitch will sound dull and won't be in tune with recorded music, other instruments, online lessons or YouTube videos.
The worst case scenario with a neglected piano is when the tuner arrives if he has to deliver the news that a piano is in such poor condition that it can't be tuned at all. This is relatively rare but it does happen, particularly with pianos that no one has attempted to tune for many years. This is especially bad news if the piano owner has recently paid for moving expenses to deliver the piano to their home.
Fortunately, this is relatively rare but it isn't uncommon for pianos that have been left without service for years to require more than the cost of an average tuning. These pianos are more vulnerable to strings breaking. Extra service charges can also arise when a piano requires repairs to the action, extra time to bring the pitch back to concert pitch or if it needs tuning pin replacement or pin tightening treatment to remedy tuning pins that are too loose to hold a proper tuning. The initial tuning visit is also a good time to assess the condition of the piano action.
Most people only think of tuning when they consider piano maintenance but the action in older pianos can be very difficult to play and can really hold a piano student back if it is badly out of adjustment or it has worn unevenly. Many old pianos have very noisy actions or due to wear, now feel so light that the player can't control the volume and may sound like they are intentionally pounding on the piano when they are trying to play more softly and with more control. A piano with hammers that are very worn with hard packed felt may sound quite metallic and “honkey tonk”. Regulating the action and hammer voicing or replacing old worn hammers can transform the touch and the tone of any piano from the least expensive upright to the most expensive concert grand.