Most transnational languages in the modern period have a system of writing, and for most such languages a standard orthography has been developed, often based on a standard variety of the language, and thus exhibiting less dialect variation than the spoken language. Sometimes there may be variation in a language's orthography, as between American and British spelling in the case of English orthography. In some languages orthography is regulated by language academies, although for many languages (including English) there are no such authorities. Even in the latter languages, a significant amount of consensus arises naturally, although a maximum of consistency or standardization occurs only when prescriptively imposed according to style guides.
Orthography is largely concerned with matters of spelling, and in particular the relationship between phonemes and graphemes in a language. Other elements that may be considered part of orthography include hyphenation, capitalization, word breaks, emphasis, and punctuation. Orthography thus describes or defines the set of symbols used in writing a language, and the rules regarding how to use those symbols.
Most natural languages developed as oral languages, and writing systems have usually been crafted or adapted as ways of representing the spoken language. The rules for doing this tend to become standardized for a given language, leading to the development of an orthography that is generally considered "correct". In linguistics the term orthography is often used to refer to any method of writing a language, without judgment as to right and wrong, with a scientific understanding that orthographic standardization exists on a spectrum of strength of convention. The original sense of the word, though, implies a dichotomy of correct and incorrect, and the word is still most often used to refer specifically to a thoroughly standardized, prescriptively correct, way of writing a language. A distinction may be made here between etic and emic viewpoints: the purely descriptive (etic) approach, which simply considers any system that is actually used--and the emic view, which takes account of language users' perceptions of correctness.
Orthographic units, such as letters of an alphabet, are technically called graphemes. These are a type of abstraction, analogous to the phonemes of spoken languages; different physical forms of written symbols are considered to represent the same grapheme if the differences between them are not significant for meaning. For example, different forms of the letter "b" are all considered to represent a single grapheme in the orthography of, say, English.
Graphemes or sequences of them are sometimes placed between angle brackets, as in ⟨b⟩ or ⟨back⟩. This distinguishes them from phonemic transcription, which is placed between slashes (/b/, /bæk/), and from phonetic transcription, which is placed between square brackets ([b], [bæk]).
The writing systems on which orthographies are based can be divided into a number of types, depending on what type of unit each symbol serves to represent. The principal types are logographic (with symbols representing words or morphemes), syllabic (with symbols representing syllables), and alphabetic (with symbols roughly representing phonemes). Many writing systems combine features of more than one of these types, and a number of detailed classifications have been proposed. Japanese is an example of a writing system that can be written using a combination of logographic kanji characters and syllabic hiragana and katakana characters; as with many non-alphabetic languages, alphabetic romaji characters may also be used as needed.
Orthographies that use alphabets and syllabaries are based on the principle that the written symbols (graphemes) correspond to units of sound of the spoken language: phonemes in the former case, and syllables in the latter. However, in virtually all cases, this correspondence is not exact. Different languages' orthographies offer different degrees of correspondence between spelling and pronunciation. English orthography, French orthography and Danish orthography, for example, are highly irregular, whereas the orthographies of languages such as Russian, German and Spanish represent pronunciation much more faithfully, although the correspondence between letters and phonemes is still not exact. Finnish, Turkish and Serbo-Croatian orthographies are remarkably consistent: approximation of the principle "one letter per sound".
An orthography in which the correspondences between spelling and pronunciation are highly complex or inconsistent is called a deep orthography (or less formally, the language is said to have irregular spelling). An orthography with relatively simple and consistent correspondences is called shallow (and the language has regular spelling).
One of the main reasons for which spelling and pronunciation deviate is that sound changes taking place in the spoken language are not always reflected in the orthography, and hence spellings correspond to historical rather than present-day pronunciation. One consequence of this is that many spellings come to reflect a word's morphophonemic structure rather than its purely phonemic structure (for example, the English regular past tense morpheme is consistently spelled -ed in spite of its different pronunciations in various words). This is discussed further at Phonemic orthography § Morphophonemic features.
The syllabary systems of Japanese (hiragana and katakana) are examples of almost perfectly shallow orthographies--the kana correspond with almost perfect consistency to the spoken syllables, although with a few exceptions where symbols reflect historical or morphophonemic features: notably the use of ? ji and ? zu (rather than ? ji and ? zu, their pronunciation in standard Tokyo dialect) when the character is a voicing of an underlying ? or ? (see rendaku), and the use of ?, ?, and ? to represent the sounds ?, ?, and ?, as relics of historical kana usage.
The Korean hangul system was also originally an extremely shallow orthography, but as a representation of the modern language it frequently also reflects morphophonemic features.
For full discussion of degrees of correspondence between spelling and pronunciation in alphabetic orthographies, including reasons why such correspondence may break down, see Phonemic orthography.
An orthography based on the principle that symbols correspond to phonemes may, in some cases, lack characters to represent all the phonemes or all the phonemic distinctions in the language. This is called a defective orthography. An example in English is the lack of any indication of stress. Another is the digraph th, which represents two different phonemes (as in then and thin) and replaced the old letters ð and þ. A more systematic example is that of abjads like the Arabic and Hebrew alphabets, in which the short vowels are normally left unwritten and must be inferred by the reader.
When an alphabet is borrowed from its original language for use with a new language--as has been done with the Latin alphabet for many languages, or Japanese Katakana for non-Japanese words--it often proves defective in representing the new language's phonemes. Sometimes this problem is addressed by the use of such devices as digraphs (such as sh and ch in English, where pairs of letters represent single sounds), diacritics (like the caron on the letters ? and ?, which represent those same sounds in Czech), or the addition of completely new symbols (as some languages have introduced the letter w to the Latin alphabet) or of symbols from another alphabet, such as the rune þ in Icelandic.
After the classical period, Greek developed a lowercase letter system that introduced diacritic marks to enable foreigners to learn pronunciation and in some cases, grammatical features. However, as pronunciation of letters changed over time, the diacritic marks were reduced to representing the stressed syllable. In Modern Greek typesetting, this system has been simplified to only have a single accent to indicate which syllable is stressed.