A tuning fork is a simple metal two-pronged fork with the tines formed from a U-shaped bar of elastic material (usually steel). When set vibrating with a blow from a mallet, and after waiting a moment to allow some high overtones to die out, a tuning fork resonates at a constant, specific pitch. The pitch that a particular tuning fork generates depends upon the length of the two prongs, with two nodes near the bend of the U. Key of A-440 [Square].