Note: Do not rely on this information. It is very old.
PP, p, the fifteenth letter of the Latin, and the sixteenth of the English, alphabet, the hard labial voiceless stop or explosive consonant formerly called the hard labial mute. The character is derived from the Phoenician pi. Though common in English, it is seldom found in words of Anglo-Saxon origin. Before m, as in topmast, the p sound is produced, not by opening the lips, but by lowering the velum pendulum, and so letting breath pass through the nose. In some words of Greek origin p is mute before n and s, as in pneumatic, psalm.