The One-Time Pad, or OTP is an encryption technique in which each character of the plaintext is combined with a character from a random key stream. Originally described in 1882 by banker Frank Miller (USA), it was re-invented in 1917 by Gilbert Vernam and Joseph Mauborgne. When applied correctly, the OTP provides a truely unbreakable cipher. It is named after the sheets of paper (pads) on which the key stream was usually printed. It also exists as One Time Tape (OTT).
OTPs like this, were commonly used for sending coded messages via a Russian spy radio set such as the R-353, often in relation to the mysterious Numbers Stations on the short wave radio bands. The OTP booklet shown here is from the internal collection of the Dutch Intelligence Agency AIVD.
Courtesy AIVD Netherlands 
Real OTP booklets are extremely rare as they were normally destroyed after use. The ones that did survive are generally in the hands of the intelligence and law enforcement agencies that used or confiscated them.