There are differing opinions over whether Kajkavian is best considered a dialect of Serbo-Croatian or a fully-fledged language of its own, as it is only partially mutually intelligible with other dialects and bears more similarities to Slovene (especially the Prekmurje dialect) than to the prestige Shtokavian dialect (which forms the basis of the national normative standards of Serbo-Croatian) in terms of phonology and vocabulary. Notable Croatian linguists consider Kajkavian to be a language in its own right, with its own established dialects and documented literature. Croatian linguist Stjepan Ivšić has used Kajkavian vocabulary and accentuation, which significantly differs from that of Shtokavian, as evidence. Furthermore, there is no clear demarcation between Slovene dialects and Kajkavian: this continuum is particularly strong along the border with Slovenian Styria, and on the upper stream of the Kolpa river, where dialects spoken on both sides of the border are sometimes indistinguishable. Thus, Kajkavian has low mutual intelligibility with Shtokavian, on which Croatia's standard language is based. Linguist Josip Silić, one of the main initiators behind the standardisation of the Croatian language, also regards Kajkavian as a distinct language by dint of its having significantly different morphology, syntax and phonology from the official Shtokavian-based standard. As of 2015, historic Literary Kajkavian has a separate language ISO 639-3 code – "kjv". Active attempts are being made by some organizations to widen its recognition and status, which has thus far included introduction of elective school subjects in Kajkavian in some parts of Croatia.