This appendix details syllabification and hyphenation rules for Finnish.
A syllable nucleus in Finnish can be a single short vowel, a long vowel or a diphthong. Any sequence of three vowels is always split up into at least two syllables. Diphthongs belong to three classes:
- I-diphthongs: any vowel + i, which is a diphthong regardless of its position
- U-diphthongs: au, eu, iu, ou, äy, ey, iy, öy, which are diphthongs (according to the standard rules) if
- word-initial (including within compounds), or
- in an open syllable (a syllable without a consonantal coda)
- opening diphthongs ie, uo, yö, which are diphthongs only if word-initial (including within compounds)
Any diphthong may however be broken either explicitly or implicitly, in which case it will not be a diphthong, but a vowel sequence with a hiatus. Explicit breaking refers to an orthographical symbol like the apostrophe (’ or '), while implicit breaking refers to optional breaking if the two vowels are broken by a -∅- (i.e. the weak grade of a consonant like -k- that may disappear altogether). For example, the genitive of haku (weak stem ha(∅)u-), haun, is usually two syllables (ha‧un), but a single syllable (haun) is also acceptable. Implicit breaks generally only apply to inflections, but not to derivations (laki > genitive lain = la‧in / lain, but laki → laiton is usually lai‧ton). Long vowels are only subject to explicit breaking.
The rule regarding U-diphthongs is relatively recent and is codified into the standard language. It is however not observed by all dialects and speakers. An example of standard syllabification with U-diphthongs: hajauttaa is ha‧ja‧ut‧taa, but hajautua is ha‧jau‧tua.
Syllables never cross word boundaries in compound words (or with suffixes that are treated as parts in a compound word, like -eleinen), but may cross morpheme boundaries with derivational affixes.
The syllabification of loanwords with consonant clusters varies. The "generally correct" way to break clusters into syllables is to do so etymologically, but using the standard Finnish rules is always acceptable (where a syllable may only begin with a single consonant, which means that a long cluster will result in a syllable ending in many consonants). For example, konstruktio may be either kon‧struk‧ti‧o or konst‧ruk‧ti‧o.
Hyphenating Finnish terms generally follows the syllabification, but there are some additional constraints:
- a syllable containing a single vowel is never left on its own line, if it can be avoided: asia cannot be broken (×a‧sia and ×asi‧a would both be highly inadvisable)
- compound words are generally broken on word boundaries (especially instead of being broken at the previous or next syllable break): viranomainen is generally viran‧omainen, not *virano‧mainen or *vir‧anomainen
- if the rule above does not apply, it is preferred that a word will not be broken such that a vowel immediately follows the break: tärkeää is generally tär‧keää, not *tärke‧ää (which is however possible if necessary)
- breaking a word at an apostrophe is not advised, but if done, the apostrophe should be omitted.
The hyphen used for hyphenation comes before the break in most cases, but comes after it if the part before the break is a noun, proper noun or phrase consisting of multiple words. This lines up with the spacing rules of the hyphen.