Variation in writing is highly frequent at both the visual and the functional levels. However, as of yet, the associated notion of allography has not been systematically described. In this article, two major types of allography are proposed: graphetic allography, conceptually comparable to allophony, depends on visual similarity and captures how concrete units are associated with visual abstractions, i.e., how three graphs in <cabana> are instances of the basic shape |a|. Graphematic allography, conceptually closer to allomorphy, does not depend on visual similarity but groups together units that share the same function, i.e., represent the same linguistic unit (phoneme, syllable, morpheme, etc.) and are complementarily distributed, meaning there exist no contexts in which they contrast. An example is the positionally conditioned alternation between |σ| vs |ς| for the Greek grapheme <σ>. By means of a number of criteria, subtypes of graphetic and graphematic allography are proposed and examples are given from different writing systems. A special case that is discussed is the complex phenomenon of capitalization. Additionally, examples of variation phenomena that are not included in the concept of allography are given, and orthographic variation is addressed as a marginal case of variation dependent on the norm rather than the system.