eenlettercode

Eenlettercode, literally “one-letter code” in Dutch, is a form of coding in which a single letter represents a particular category, label, or item within a system. Such codes are used to simplify data entry, storage, and retrieval, and they are common across science, technology, and administration. The exact meaning of a given letter is defined in the relevant documentation or standards for that domain.

One of the best-known examples is the set of single-letter amino acid codes used in biochemistry and molecular biology to represent the 20 standard amino acids in protein sequences. A, C, D, E, F, G, H, I, K, L, M, N, P, Q, R, S, T, V, W, Y correspond to Alanine, Cysteine, Aspartic acid, Glutamic acid, Phenylalanine, Glycine, Histidine, Isoleucine, Lysine, Leucine, Methionine, Asparagine, Proline, Glutamine, Arginine, Serine, Threonine, Valine, Tryptophan, and Tyrosine. Additional letters (B, Z, X) are used to indicate ambiguity or unknown residues in different databases.

In other fields, one-letter codes label states, categories, or types in concise data schemes. They help researchers and practitioners compare records, but they require clear documentation, because a single letter can be ambiguous without context. In data standards, a one-letter code is typically defined in a codebook, along with its allowed values, scope, and applicability.

See also: multi-letter codes, abbreviated notation, protein sequences.