Fortis and lenis
Fortis (Latin "strong") and lenis ("weak") are linguistic terms.
In a wide sense, they refer to the opposition of consonants such as p, t vs. b, d. In a narrow sense, fortis refer to consonants such as p, t pronounced with tenseness (more muscular tension) and lenis to consonants such as b, d pronounced without.
History and use of the terms
These terms were already used in 19th century German linguistics, in order to describe languages such as southern German where consonants such as b, d are voiceless but nonetheless different from p, t. The terms are only seldom used in current linguistics.
Opposition of p, t versus b, d
Normally, the opposition of consonants such as p, t vs. b, d is described in terms of voiceless consonants vs. voiced consonants. There are languages where this is indeed the only feature that distinguishes them (e.g. French: IPA [p, t] vs. [b, d]). In many languages, however, the phonetic voice is only one of several features that constitutes this opposition (e.g. English: IPA [pʰ, tʰ] vs. [b, d] ). There are even languages where the phonetic voice is not a distinctive feature of these pairs at all (e.g. southern German, Danish or Mandarin: IPA [pʰ, tʰ] vs. [b̥, d̥]).
The terms fortis and lenis (in the wide sense) apply to this opposition regardless of whether it's only an opposition of voice or not. Therefore, it allows one to speak in the same terms of French, English, or southern German consonants.
Consequently, they refer to a bundle of articulatory features which have different distributions in different languages. Not all of them need to be present in a particular language:
- voice (voiceless vs. voiced)
- length
- aspiration
- tenseness
Tenseness
It is commonly said that tenseness is what distinguishes fortis and lenis in the narrow sense: In the articulation of the fortis, more muscular energy is supposedly used. However, this has never been demonstrated.
In Korean, a higher fundamental frequency of vowels following certain 'tense' consonants is thought to be a result of increased muscular tension in the vocal cords, a phonation called stiff voice. However, in Swiss German, no possible acoustical correspondent of the assumed tenseness has been found. Consequently, it is debated whether the Swiss German opposition is really based on different muscular tension, and not on gemination.
A few languages have been claimed to distinguish consonants solely by tenseness or laxness: some dialects of Adyghe distinguish voiceless tense stops from voiceless lax stops, and both are separate phonemes from voiced lax stops.
Regardless, articulatory strength varies from speaker to speaker and language to language. The Ewe language, for example, which contrasts a voiceless bilabial fricative /ɸ/ and a voiceless labiodental fricative /f/, pronounces the /f/ markedly more strongly than is the case in most languages. This helps differentiate what would otherwise be an exceedingly subtle distinction. Phonetically, a diacritic from the Extended IPA can be used to indicate this strong articulation: [ɸ] vs. [f͈].
In the Mixe-Zoquean language Mixe the distinction between consonants desribed as fortis and lenis has been demonstrated to be one of quantity: fortis consonants are pronounced longer than their lenis counterparts, and they are also not prone to voicing in voiced environments such as the lenis consonants are.