Trills are very different from flaps. Whereas with a flap (or tap), a specific gesture is used to strike the active articulator against the passive one, in the case of a trill the articulator is held in place, where the airstream causes it to vibrate. Usually a trill vibrates for 2-3 periods, but may be up to 5, or even more if geminate. However, trills may also be produced with only a single period. While this might seem like a flap, the articulation is different; trills will vary in the number of periods, but flaps do not.
The bilabial trill is uncommon. The coronal trill is most frequently alveolar , but dental and postalveolar articulations and also occur. A retroflex trill found in Toda has been transcribed (that is, the same as the retroflex flap), but might be less ambiguously written . One other trill has been reported, an epiglottal trill. Epiglottal consonants are often allophonically trilled, and in some languages the trill is the primary realization of the consonant. There is no official symbol for this in the IPA, but occasionally [я] will be used. There are also vowels accompanied by epiglottal trill, called strident.
The Czech language has two contrastive alveolar trills. In one of these the tongue is raised, so that there is audible frication during the trill, sounding rather like a simultaneous and . A symbol for this sound, , has been dropped from the IPA. It is now generally transcribed as a raised r, . Liangshang (Cool Mountain) Yi has two "buzzed" or fricative vowels, written , which may also be trilled, .
