West Frisian differs from Dutch in vocabulary, in grammar, and in vowel system. 1 2 3 4

Like Dutch it has a four-step system of shorts: y           oe   ú   i       o   u       e   ò               a

and of longs and diphthongs:                               ee   oo               eu               ei/ij   au/ou   ui         aa

In addition, however, West Frisian has a number of 'curled' diphthongs: ie           oe   ue   ea       oa           ê   â/ô

and its 'broken' counterparts: iè           uo   ju   eà       oà                                           as well as two 'stretched' vowels: ii           û

With fourteen vowel phonemes more than Dutch, one may call this a 'rich' vowel system.

It must be stated, that other Frisian dialects have quite different, but also rather rich vowel systems. For a possible explanation of that 'wealth' see A phonematic catastrophy.