Absolute pitch (AP), or perfect pitch, is the ability to name or reproduce a tone without reference to an external standard.

Perfect pitch people can:

• Identify by name individual pitches (e.g. A, B, C♯) played on various instruments
• Name the key of a given piece of tonal music just by listening (without reference to an external tone)
• Identify and name all the tones of a given chord or other tonal mass
• Accurately sing a given pitch without an external reference
• Name the pitches of common everyday noises such as car horns
• Identify the numerical value in hertz of a given note.

^wikipedia

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