Slavic and Germanic languages have considerably more sounds (phonemes) than Latin. It caused technical problems when at the beginning of the second millennium the Latin alphabet was employed for writing these vernaculars. The initial makeshift strategy was to use two or three letters (diagraphs and trigraphs) for the extra sounds. Czech was the first written language among Catholic and Protestant Slavs. Not surprisingly in the 14th century the diagraph [cz] was employed for writing the phoneme /tʃ/, the diagraph [rz] for /r̝/, or the diagraph [ſſ] for /ʃ/. With his 1406 work De orthographia Bohemica, Jan Hus changed this system by introducing diacritical letters for the extra Slavic phonemes. As a result, in today’s Czech orthography each phoneme is reflected by a single letter, for instance, /tʃ/ is written as [č], /r̝/ as [ř], or /ʃ/ as [š]. The sole exception to this rule is the grapheme [ch] retained to denote the sound /x/.
But Jan Hus was also the ideological proponent of the renewal of the Catholic Church. For his efforts he was burnt at the stake in 1415. This event led to the outbreak of the Hussite Wars (1414-1434) and the rise of the first Reformation, a whole century before Martin Luther. The influence of Hussitism continued in the Czech lands (especially in Bohemia) until the stalemate between Protestants and Catholics, in which the Thirty Years’ War ended in 1648. Meanwhile Protestants (Hussites) had been expelled from the Czech lands, now under the Catholic Habsburgs’ rule.
During the time of the religious wars the new Czech orthography was seen by Catholics as ‘heretical,’ due to its connection to the person of the vilified Jan Hus. When the Polish language began to be committed to paper in the 15th and 16th centuries, obviously the Old Czech spelling systems with diagraphs and trigraphs was selected in preference to the Hussite one. Poland-Lithuania was one of the leading monarchies in the Catholic camp. As a result, to this day the Polish language employs this Old Czech spelling. Hence, [cz] is used for writing the phoneme /tʃ/, the diagraph [rz] for /ʐ/, or the diagraph [sz] for /ʃ/.
Nowadays, all the Slavic languages written in Latin letters use either the Old or New (Hussite) Czech spelling. Interestingly, only the Polish language sticks to the former. Although in the past the majority of Latin script-based Slavic languages were loyal this solution, nevertheless all of them dropped the Old Czech spelling for the Hussite one in the course of the 19th century. Meanwhile, the Hussite spelling had lost its religious connotations and become known as ‘Czech spelling.’ In turn, the old Czech spelling was dubbed ‘Polish spelling.’ Diagraphs and multigraphs are also employed to this day for writing German and Hungarian.
Many Slavic languages were standardized in the Habsburg lands in reaction to the perceived domination of German, Hungarian, or Polish culture. As a result, the choice of the (new) Czech spelling appeared to be a form or symbol of desired ethnocultural emancipation. Hence, nowadays Bosnian, Croatian, Czech, Montenegrin, Slovak, Slovenian and Sorbian are written in the ‘new Czech manner,’ while only Polish in the ‘old Czech manner.’ Until the turn of the 20th century the non-Slavic language of Lithuanian and Belarusian in publications for Uniates had employed the Polish spelling. But Lithaunian and Belarusian activists felt their respective cultures to be threatened by Polonization, and in order to forestall this danger adopted the (new) Czech spelling for writing their languages.