Fortis is a term commonly used in the Celtic linguistics literature to refer to sounds in a phoneme inventory that are tense. In Gaelic, the fortis/lenis (tense/lax) distinction is part of a four way contrast in sonorants /l, n, r/. Tense consonants are indicated by capital letters (/L/), and 'slender' consonants are indicated by a following apostrophe (/l'/). See Sounds of Scottish Gaelic for a closer look at this distinction.
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- Crystal, David (2008). A Dictionary of Linguistics and Phonetics Sixth Edition. Blackwell Publishing Ltd.
- Crystal, David. A Dictionary of Linguistics and Phonetics. The language library. Oxford, UK: Blackwell, 1997.