Czech orthography is a system of rules for proper formal writing (orthography) in Czech. The earliest form of separate Latin script specifically designed to suit Czech was devised by Czech theologian and church reformist Jan Hus, the namesake of the Hussite movement, in one of his seminal works, De orthographia bohemica (On Bohemian orthography). The modern Czech orthographic system is diacritic, having evolved from an earlier system which used many digraphs (although one digraph has been kept - ch). The caron is added to standard Latin letters to express sounds which are foreign to Latin. The acute accent is used for long vowels. The Czech orthography is considered the model for many other Balto-Slavic languages using the Latin alphabet; Slovak orthography being its direct revised descendant, while the Serbo-Croatian Gaj's Latin alphabet and its Slovene descendant system are largely based on it. All of them make use of similar diacritics and also have a similar, usually interchangeable, relationship between the letters and the sounds they are meant to represent. Alphabet The Czech alphabet consists of 42 letters. The letters Q, W, and X are used exclusively in foreign words, and the former two are replaced with KV and V once the word becomes “naturalized” (assimilated into Czech); the digraphs dz and dž are also used mostly for foreign words and are not considered to be distinct letters in the Czech alphabet. Orthographic principles Czech orthography is primarily phonemic (rather than phonetic) because an individual grapheme usually corresponds to an individual phoneme (rather than a sound). However, some graphemes and letter groups are remnants of historical phonemes which were used in the past but have since merged with other phonemes. Some changes in the phonology have not been reflected in the orthography. Voicing assimilation All the obstruent consonants are subject to voicing (before voiced obstruents except ) or devoicing (before voiceless consonants and at the end of words); spelling in these cases is morphophonemic (i.e. the morpheme has the same spelling as before a vowel). An exception is the cluster , in which the is voiced to only in Moravian dialects, while in Bohemia the is devoiced to instead (e.g. shodit , in Moravia ). Devoicing changes its articulation place: it becomes . After unvoiced consonants is devoiced: for instance, in 'three', which is pronounced . Written voiced or voiceless counterparts are kept according to the etymology of the word, e.g.