Possible Clues:

• Important piece at apex of arch
• Central block at the top of an arch
• Chief brick in an arch
• Cops perhaps find openers on pitch
• The chief element in the sound of a piano?
• Wedge-shaped piece in a bridge's arch
• Musical sequences must have musical quality in principle
• Vital principle of part of piano sound
• Locking piece for arch
• Two piano features on which all else depends