Orthography refers to the rules for writing a language, such as conventions of spelling and punctuation. In an alphabetic script, this also includes its grapheme-phoneme (letter-sound) correspondences. Sometimes writing system is defined in the same way as 'orthography', but this also has the meaning of how a set of signs is used to write a language systematically (e.g. alphabetically or syllabically).
The Roman alphabet serves to illustrate the difference between these terms. It is a single script and a single writing system, but each language it is used for has a different orthography. Therefore it is appropriate to discuss 'English orthography' and 'French orthography' as different, though they use the same script. For example, both use the 'digraph' (two letters in combination) <ch>, but in French it represents /ʃ/ as in chat 'cat' while in English it can be /tʃ/, /k/ or /ʃ/ as in church, chemistry and champagne. Orthography also covers differences between varieties of the same language, such as the presence or absence of <u> in words such as colour in British and American English. Some languages, such as Japanese, use more than one kind of writing system in their overall written language system; in these cases, orthographic rules determine which system is used, e.g. for example, katakana (カタカナ) is typically used for words of recent foreign origin, or to emphasise words or phrases. In English orthography, this is achieved by italics, bold or CAPITAL letters.