Long vowels /iː, uː, eː, øː, oː, ɛː, ɔː, aː/ tend to be slightly diphthongized to [iːj, uːw, eːj, øːj, oːu, ɛːə, ɔːə, aːə].[more recent source needed] This is especially true of word-final /eː, øː, oː/, which are regularly diphthongized.
On average Frisians use a high number of long vowels. To reduce the length of speech there are two systems to reduce sound length:
The first is the new Frisian breaking. New Frisian breaking reduces the length of a long vowel by replacing it with a consonant and a vowel or semi vowel. Breaking pairs are ie-ji/i, ue-uo, oa-ua and dei-je/dje.
The second system is article and suffix reduction to glottal stops. Both definite articles (de (/də/) and it (/ət/)) and the indefinite article (in (/ən/)) may undergo article reduction. Popular suffixes for reduction include -en (/ən/) and -t (/ət/). "it tinken" (the act of thinking) for example may be reduced from /ət tɪŋ͡kən/ to [ʔtɪŋ͡kʔ]. This however is an extreme reduction and does not occur in most dialects. More common is removing the schwa (/ə/) and in case of a reduced article following a reduced dental suffix one of the /t/'s is dropped. In Standard Frisian writing this phenomenon can be represented by replacing the character representing the schwa by an apostrophe.
The use of both systems varies from dialect to dialect and in the standard language there are irregularities whether to write broken vowels in their broken or unbroken form.