Phonology
Lengthened fricatives, nasals, laterals, approximants, and trills are simply prolonged. In lengthened stops, the obstruction of the airway is prolonged, delaying release. That is, the "hold" is lengthened. Long consonants are usually around one and a half or two times as long as short consonants, depending on the language.
In some languages, e.g., Italian, Swedish, Faroese, Icelandic and Ganda, consonant length and vowel length depend on each other. That is, a short vowel within a stressed syllable almost always precedes a long consonant or a consonant cluster, whereas a long vowel must be followed by a short consonant. In Classical Arabic, a long vowel was lengthened even more before permanently geminate consonants, however this is no longer exhibited in varieties of colloquial Arabic or even Modern Standard Arabic.
In other languages, such as Finnish, consonant length and vowel length are independent of each other. In Finnish, both are phonemic, such that taka /taka/ "back", takka /takːa/ "fireplace", taakka /taːkːa/ "burden", and so forth are different, unrelated words; this distinction is traceable all the way back to Proto-Uralic. Finnish consonant length is also affected by consonant gradation. Another important phenomenon is that sandhi produces long consonants to word boundaries from an archiphonemic glottal stop, for example |otaʔ se| → /otasːe/ "take it!"
Distinctive consonant length is usually restricted to certain consonants. There are very few languages that have initial consonant length; among them are Pattani Malay, Chuukese, a few Romance languages such as Sicilian and Neapolitan, and many of the High Alemannic German dialects (such as Thurgovian). Some African languages, such as Setswana and Ganda, also have initial consonant length—in fact, initial consonant length is very common in Ganda and is used to indicate certain grammatical features. In spoken Finnish and in spoken Italian, long consonants are produced between words by sandhi effects.
Among stops and fricatives, in most languages only voiceless consonants occur geminated.
The reverse of gemination is the process in which a long consonant is reduced to a short one. This is called degemination. This is a pattern observed in Baltic-Finnic consonant gradation, where the strong grade (often, but not necessarily nominative) form of the word is degeminated into a weak grade (often all other cases) form of the word, e.g. taakka → taakan (burden, of the burden).
Read more about this topic: Gemination