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The long continuous hinge on the lid of a piano is so characteristic that it's even named a "piano hinge" after this use. But why do pianos have such hinges, when a set of separate hinges, such as you'd find on a wooden cabinet or jewellery box, would be cheaper to make and easier to fit?

Is it just to make it look nice as a premium piece of woodwork? Is it to increase the rigidity of the structure or prevent unwanted vibrations in the joint? Or is it for some other reason to do with the sound or the construction of the instrument?

  • I don't think it would be "easier to fit" separate hinges. If there were more than two hinges, you would need to align them all accurately. A piano hinge is self-aligning, unless you are so clumsy that you bend it while screwing it to the wood. – user19146 Sep 17 '15 at 21:25
  • @alephzero Door hinges are not hard to align, even when three are used. They're also quick to countersink and screw, and can be fitted one at a time. A piano hinge needs a much bigger area chiselled to countersink, more screw holes, and the continuous side of it needs to be fitted in one go. – Dan Hulme Sep 17 '15 at 21:31
  • You're talking about an upright piano, not a grand, correct? The lid on a grand piano is usually attached with a few small hinges so that it is easily removable for performance of duo piano music. – NReilingh Oct 4 '15 at 3:44
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You need to consider the total weight of the piano (say 300 pounds / 135 kg for a small upright, up to 800 pounds / 350 kg for a full size grand) compared with a typical piece of wood furniture.

When a piano is moved about by "non-professionals", there is the possibility that a lot of weight gets transmitted through the hinges on the lid and the fallboard (the part of the case that covers the keys).

The lid of a grand piano is quite heavy in itself, and it has two hinge lines - one along the left hand side to raise and lower it, and another to fold back the front part when playing with the lid down.

Obviously you don't want the hinges making rattling noises, but I think the main reason for continuous hinges is simply "good mechanical design practice".

This "overdesigned" style of construction goes back to the earliest Italian harpsichords, which were much lighter (typically 50 pounds) but designed to survive being transported on the back of a pack-horse, or even on the back of a human if necessary.

  • Also, transported "on the side", with the hinge down, needing extra strength. – yo' Sep 18 '15 at 22:01
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Piano hinges don't put much force on the individual screws and they can (depending on the build) be taken apart by pulling out the axle. The metal is pliable enough that there is nothing that would make a permanent buzz or noise or transmit a considerable amount of sound.

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    The fact that the lid can be removed quickly and easily has to make this the best answer.+1. – Tim Sep 17 '15 at 17:52
  • There isn't much reason to completely remove and replace the lid of a grand except in a recording studio, but the ease of initial assembly is a plus point for the piano hinge compared with several individual hinges. – user19146 Sep 17 '15 at 21:29
  • @alephzero - it would probably be removed for transport. – Tim Sep 21 '17 at 11:33
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I think that an important thing to consider is not only the weight, but also the price. With door or funritnure door, the hinge makes a substantial difference in price. But a typical grand is in tens of thousands of euros or dollars, and a small piece of metal does not make a difference really. So if you discuss whether you save couple cents by reducing the mechanical quality, you realize it's not worth it.

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