Mnemonics are memory aids that may appear to be unusual or artificial, but are based on the basic principles of learning and memory. Mnemonic techniques and systems have been used for at least 2,500 years, but have been studied experimentally for less than 40 years. Mnemonic techniques can be verbal (using words) or visual (using visual imagery). Verbal mnemonics include first-letter mnemonics (acronyms and acrostics), rhymes and songs, and stories. The keyword mnemonic involves two steps, one verbal (constructing a concrete word to substitute for an abstract term) and one visual (a visual image associating the substitute word with the meaning of the abstract term). Visual mnemonics include the use of visual imagery accompanying some verbal mnemonics, and the mnemonic systems described below. Interaction, vividness, and bizarreness can help make visual associations effective. Mnemonic systems are mental filing systems that are more general-purpose than mnemonic techniques. The loci system involves pre-memorizing visual images of familiar locations in a natural order, then associating images of the to-be-remembered items with the locations. The peg system uses pegwords that represent numbers, which are then used in the same manner as the locations in the loci system, except that direct retrieval as well as sequential retrieval is possible. The phonetic system represents the digits 0–9 by consonant sounds, which are used to construct keywords to represent numbers; these keywords are used in the same manner as the pegwords in the peg system, except that more keywords can be constructed and numerical information can also be remembered.