Dutch phonology is similar to that of other West Germanic languages, especially Afrikaans and West Frisian. While the spelling of Dutch is officially standardised by an international organisation (the Dutch Language Union), the pronunciation has no official standard and relies on a de facto standard documented in reference works such as The Phonetics of English and Dutch by Beverley Collins and Inger M. Mees, The Phonology of Dutch by Geert Booij, Dutch by Carlos Gussenhoven, Belgian Standard Dutch by Jo Verhoeven or pronunciation dictionaries such as Uitspraakwoordenboek by Josée Heemskerk and Wim Zonneveld.