In loanwords, the stops k, p, and t and less regularly the sounds s and f are often doubled in Finnish, in situations where the original language has a single consonant. The word “cent” is sentti in Finnish, even though most European languages show no sign of doubling the “t” in this word.
There has been and there still is variation in the doubling of consonants in new loanwords, both in pronunciation and in spelling. Although the rules are mostly well-defined and generally followed nowadays, there are words where a single consonant is read as double or vice versa.
A double consonant written before a consonant is always read as single, as in English, e.g. attraktio [atraktio] (attraction). Such a double consonant usually comes from a Latin origin. There is no difficulty in pronunciation, but if you know a word in spoken form only, or from another language, the Finnish spelling cannot always we inferred correctly. For example, adressi (petition; scroll) is written with one d just because Swedish has “adress”.
The doubling issue discussed here is an opposite phenomenon: only a single consonant is written and pronounced in the original language, but Finnish has a double consonant at least in pronunciation.
In loanwords, a consonant following a long vowel, a diphthong, a liquid consonant (l, r), or a nasal consonant (m, n) is usually pronounced doubled. In old loanwords, this is reflected in spelling, e.g. kaappi (from Swedish “skåp”), manttaali (from Swedish “mantal”), marssi (from Swedish “marsch”). In early 20th century, after heavy debate, a different spelling policy was adopted for newer loanwords. A compromise was made between two logical solutions.
It was decided to express the doubling when the consonant is followed by a final i or a Finnish suffix or both, but not otherwise. Therefore, the standard spelling is satraappi (satrap) but kampanja (campaign), even though the latter is normally pronounced kamppanja and also often written that way. Often the distinction between old and new loanwords is rather arbitrary; for example, samppanja (champagne) is the standard spelling. Note that rule means that the double consonant, if it is a stop, usually participates in consonant gradation, e.g. satraappi : satraapin.
The compromise has some additional exceptions: before eri, eli, ori the doubling is standard and expressed in writing: eetteri, tuberkkeli, senaattori. In some words in these categories, doubling is not common in speech (e.g. kraatteri, preettori).
The doubling does not apply, in pronunciation or spelling, to stops (p, t, k) when the original language has a voiced stop (b, g, d). For example, the ending -lanti in country names comes from Swedish “-land”. In contrast, the Atlantic Ocean is Atlantti.
Common pronunciation often doubles the consonant even when it should be single according to the rules described above, e.g. beeta [beetta], delta [deltta], kvartaali [kvarttaali], andante [andantte], though this is far from systematic. When it happens, there is consonant gradation in pronunciation but not in spelling, e.g. beeta [beetta] : beetan [beetan].
The compromise was originally made to establish the written form. Only later was it decided that the pronunciation should follow the spelling. Yet, after about a hundred years after the compromise, doubling is still normal pronunciation, under the conditions specified above, for the stops k, p, and t. Examples (with common pronunciation in brackets): sampoo [samppoo], temperamentti [tempperamentti], internetiin [intternettiin]. Most Finns probably use such pronunciations without thinking and without realizing the difference between the spelling and the pronunciation. Yet, spellings that correspond to such pronunciation are commonly recognized as erroneous.
The situation is somewhat different for s and f: doubling is common, e.g. in marsilainen [marssilainen] (Martian, from the name Mars) and graafinen [graaffinen], but less common than for the stops and possibly regarded as vulgar. However, when f appears after a consonant, doubling is regular. Official spellings are golfin (of golf) and surfata (to surf), but practically everyone says [golffin] and [surffata]; and now surffata is allowed as an alternate spelling.
There are many further complications in the phenomenon. In some words, like alkoholi, doubling, [alkkoholi], may sound odd to many people; in some words, like beeta, it would probably sound odd not to pronounce [beetta]. The variation is usually not described in dictionaries.
The rules cause difficulties even to educated native speakers especially in words that are apparently related, but should be treated differently according to the rules, e.g. hierarkia (hierarchy) versus hierarkkinen (hierarchic). The former has single k before the foreign-origin ia ending, whereas the latter has doubled kk, since it is followed by i and the Finnish suffix nen.
Foreign words that end with k, p, or t in the basic form have that consonant doubled in inflected forms, except that it participates in consonant gradation. To put this in a perhaps better way, these words are inflected as if their basic forms ended with kki, ppi, or tti. However, according to the rules, the double consonant is written with one letter.
For example, pop is inflected as if it were poppi, but written with p instead of pp: pop : popia [poppia] : popina [poppina] : popiin [poppiin] : popin [popin] etc. In practice, in singular forms, only the partitive, essive, and illative cases have a double kk, pp, or tt pronounced in them; in other cases, consonant gradition makes the stop short. According to language guides, the doubling described here may or may not take place, but in reality, it is part of normal pronunciation – [popia] would sound artificial.
For one-syllable words ending with s, doubling is also common, in all inflected forms, e.g. fis [fis] : fisiä [fissiä] : fisin [fissin]. However, here it may actually vary with single-consonant pronunciation, especially when the s appears after a consonant, e.g. Mars [mars] : Marsin [marssin] ∼ [marsin]. In-between pronunciation, with a half-long consonant, is probably common. However, abbreviations ending with s and pronounced as one-syllable words have the [s] sound regularly doubled, e.g. SAS [sas] : SASin [sassin]. Some abbreviations may be pronounced without doubling when speaking formally, e.g. HUS [hus] : HUSin [hussin] ∼ [husin].
Other consonants may also be doubled after a stressed syllable with a short vowel, e.g. ROM [rom] : ROMin [rommin] ∼ [romin]. This may also happen in foreign, especially French names, if they are pronounced with stress on last syllable, e.g. Laval [lavál] : Lavalin [lavállin].
Language rules require that the doubling described here must not be reflected in writing. However, for some words, doubling is now a permitted alternative, e.g. pop : popia ∼ poppia.
The background is that the doubling described in this section is applied in Swedish as spoken in Finland, at least in the Helsinki area. When words were adopted from it into Finnish, it was natural to retain both their pronunciation and their spelling, even though this deviated from Finnish orthography. Swedish as spoken Finnish, on the other hand, has been phonetically affected by Finnish. In Sweden, a word like kantarell is pronounced so that the t sounds emphatic, mainly due to the phonetic features in the change from n to t and from t to a. Swedish as spoken in Finland lacks such features and tends to compensate for this by doubling the t, to keep t sufficiently different from d.
An opposite phenomenon is that a written double consonant is often pronounced as short in words like assistentti [asistentti], penisilliini [penisiliini], and renessanssi [renesanssi]. In a word like penisilliini, such pronunciation is regular and can be observed from secondary stress: [pénisilíini] versus [pénisílliini], which we would have if the ll were pronounced long.
The reason for the shorttening is that the corresponding Swedish word has a written double consonant after an unstressed vowel, “assistént”, “penicillín”, “renässáns”, and in such a context a consonant is pronounced single in Swedish.