Hyphens in compounds

Usually a closed compound is written as a single word without any character between the parts. However, a hyphen is used inside a word in the following situations:

• The second part starts with the same vowel as the first part end with, e.g. linja-auto (bus). Note that a hyphen is not used between identical consonants, e.g. syyssateet, consisting of the parts syys and sateet.
• The compound is copulative, as explained later, e.g. suomalais-ruotsalainen (Finnish–Swedish).
• Either of the parts is a proper name, e.g. Helsingin-matka (trip to Helsinki). However, some of such compounds are written without a hyphen and in all lower case, since they have turned to common words (appearing in dictionaries), e.g. dieselmoottori (diesel engine), ranskanleipä (French bread). The hyphen is omitted if a new word is derived from a compound, e.g. Etelä-Eurooppa (South Europe) but eteläeurooppalainen (South European).
• The first part is written with digits or is a special character, e.g. 42-miehinen (with 42 men), @-merkki (@ sign).
• The first part is an abbreviation, e.g. EU-jäsenyys (EU membership). However, if it is an abbreviation that ends with a period and is meant to be read as the full word, no hyphen is used, according to current rules, e.g. puh.joht. = puheenjohtaja (chairman).
• The first part is a foreign word that makes the word difficult to read without a hyphen, e.g. go-peli (the game of go), par-tulos (par result). In practice, the parts might be well known to the intended audience, but the hyphen makes the structure more evident, so this rule might be seen as a special case of the following.
• In some words to indicate the structure of the word and to avoid misunderstandings. This is a vague rule and rarely applied, but it makes it possible, in principle, to distinguish e.g. between the compound laulu-ilta (song evening) and the non-compound lauluilta (ablative plural of laulu “song”).

A hyphen is also used at the start or end of a word in some contexts where the word is part of a larger expression. For example, etunimi ja sukunimi (first name and surname) can be written and pronounced etu- ja sukunimi. The hyphen does not affect pronunciation; it just indicates visually that here etu is not an independent word but part of a compound, just so that the second part, here nimi, is omitted, or more exactly shared with the next word. The term “suspended compound” is sometimes used about such constructs. Similarly, the first part of a compound may be shared with the preceding word the same way, e.g. syntymäaika ja -paikka = syntymäaika ja syntymäpaikka. In expressions like this, the part -paikka is pronounced with a main stress, even though it is logically a second part of a compound and would otherwise have secondary stress only.

Special rules apply when a part of a compound consists of two (or more) words, as in Alfa Romeo -autot. see section Open compound as part of closed compound.