The instrument
Sound post
The sound post is a small wooden dowel wedged inside a string instrument between the top and back, transmitting vibration and holding the body under string tension.
It stands under the treble foot of the bridge, held in place by friction alone. Moving it by a fraction of a millimetre audibly changes the instrument's response and balance, which is why the French call it the soul of the instrument and why adjusting it is a luthier's job rather than an owner's.
If a sound post falls, which can happen if the strings are all removed at once, the instrument should be taken to a luthier and not played, since the top is no longer supported against the bridge's downward pressure. This is the practical reason string players change strings one at a time.
Related terms
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