A flashcard or flash card is a set of cards bearing information, as words or numbers, on either or both sides, used in classroom drills or in private study. One writes a question on a card and an answer overleaf. Flashcards can bear vocabulary, historical dates, formulas or any subject matter that can be learned via a question and answer format. Flashcards are widely used as a learning drill to aid memorization by way of spaced repetition.
Flashcards exercise the mental process of active recall: given a prompt (the question), one produces the answer. Beyond the content of cards, which are collected in decks, there is the question of use – how does one use the cards, in particular, how frequently does one review (more finely, how does one schedule review) and how does one react to errors, either complete failures to recall or mistakes? Various systems have been developed, with the main principle being spaced repetition – increasing the review interval whenever a card is recalled correctly.
Physical flashcards are two-sided; in some contexts one wishes to correctly produce the opposite side on being presented with either side, such as in foreign language vocabulary; in other contexts one is content to go in only one direction, such as in producing a poem given its title or incipit (opening). For physical flashcards, one may either use a single card, flipping it according to the direction, or two parallel decks, such as one English-Japanese and one Japanese-English. For electronic flashcards, cards going in the opposite direction can easily be produced, and may be treated either as two unrelated cards, or being related in some way, as in the program Anki, which enforces a minimal time spacing between opposite sides of a card. They have a number of uses that can be very simple or very elaborate for the person to memorize.
There are various systems for using flashcards, many based around the principle of spaced repetition – reviewing information at increasing intervals. Manually managing interval length can add greatly to the overhead of using flashcards: the Leitner system is a simple spaced repetition system designed for paper flashcards, based on a small number of boxes and a simple algorithm, while the SuperMemo algorithms are more complicated, tracking each card individually, and designed for implementation by computer.
Three-sided cards 
Physical flashcards are necessarily two-sided. A variant, found in electronic flashcards, is what is known as a three-sided card. This is a particular kind of asymmetric two-sided card; abstractly, such a card has three fields, Q, A, A*, where Q & A are reversed on flipping, but A* is always in the answer – the two "sides" are thus Q/A,A* and A/Q,A*. Concretely, these are most used for learning foreign vocabulary where the foreign pronunciation is not transparent from the foreign writing – in this case the Question is the native word, the Answer is the foreign word (written), and the pronunciation is always part of the answer (Answer*). This is particularly the case for Chinese characters, as in Chinese hanzi and Japanese kanji, but can also be used for other non-phonetic spellings, including English as a second language.
A Chinese-English example, for learning the word 人 (rén, person):
- Q: person
- A: 人, rén
- Q: 人
- A: rén, person
- Q: enough
- A: assez
The purpose of three-sided cards is to provide the benefits of two-sided cards – ease of authoring (enter data once to create two cards), synchronized updates (changes to one are reflected in the other), and spacing between opposite sides (so opposite sides of the same card are not tested too close together) – without the card needing to be symmetric.
One can generalize this principle to an arbitrary number of data fields associated with a single record, with each field representing a different aspect of a fact or bundle of facts.
There is a wide range of software (including open source and online services) available for creating and using virtual flashcards as an aid to learning.
Paper flashcards have been used since at least the 19th century, with Reading Disentangled (1834), a set of phonics flashcards by English educator Favell Lee Mortimer being credited by some as the first flashcards. Previously, a single-sided hornbook had been used for early literacy education.
The Leitner system for scheduling flashcards was introduced by German scientific journalist Sebastian Leitner in the 1970s, specifically his 1972 So lernt man lernen. Der Weg zum Erfolg (How to learn to learn), while the SuperMemo program and algorithm (specifically the SM-2 algorithm, which is the most popular in other programs) was introduced on December 13th, 1987 by Polish researcher Piotr Woźniak.
- Adding images, sounds, mathematical formulas, and three-sided cards on The Mnemosyne Project
- The Clumsiest People in Europe: Or, Mrs. Mortimer's Bad-Tempered Guide to the Victorian World, Favell Lee Mortimer, foreword by Todd Pruzan, 2006 edition, p. 5
- So lernt man lernen. Der Weg zum Erfolg (How to learn to learn), Freiburg i. Br. 1972/2003, ISBN 3-451-05060-9
- 3. Account of research leading to the SuperMemo method, 3.1. The approximate function of optimal intervals and 3.2. Application of a computer to improve the results obtained in working with the SuperMemo method, P. A. Wozniak, Optimization of learning, Master's Thesis, University of Technology in Poznan, 1990.