by allowing historical linguists to define the notion of regular correspondence by the comparative method. Each sound change is limited in space and time and so it functions in a limited area (within certain dialects) and for a limited period of time. For those and other reasons, the term "sound law" has been criticized for implying a universality that is unrealistic for sound change. A sound change that affects the phonological system or the number or the distribution of its phonemes is a phonological change. Principles The following statements are used as heuristics in formulating sound changes as understood within the Neogrammarian model. However, for modern linguistics, they are not taken as inviolable rules but are seen as guidelines. Sound change has no memory: sound change does not discriminate between the sources of a sound. If a previous sound change causes X,Y > Y (features X and Y merge as Y), a new one cannot affect only an original X. Sound change ignores grammar: a sound change can have only phonological constraints, like X > Z in unstressed syllables. For example, it cannot only affect adjectives. The only exception to this is that a sound change may or may not recognise word boundaries, even when they are not indicated by prosodic clues. Also, sound changes may be regularized in inflectional paradigms (such as verbal inflection), in which case the change is no longer phonological but morphological in nature. Sound change is exceptionless: if a sound change can happen at a place, it will. It affects all sounds that meet the criteria for change. Apparent exceptions are possible, because of analogy and other regularization processes, another sound change, or an unrecognized conditioning factor. That is the traditional view expressed by the Neogrammarians. In past decades, however, it has been shown that sound change does not necessarily affect all possible words. However, when a sound change is initiated, it often eventually expands to the whole lexicon. For example, the Spanish fronting of the Vulgar Latin [g] (voiced velar stop) before [i e ɛ] seems to have reached every possible word. By contrast, the voicing of word-initial Latin [k] to [g] occurred in colaphus > golpe and cattus > gato but not in canna > caña. See also lexical diffusion. Sound change is inevitable: All languages vary from place to place and time to time, and neither writing nor media prevents that change.