Across languages, there is a tendency to avoid length contrasts in the most vowel-like consonant classes, such as glides or laryngeals. Such gaps could arise from the difficulty of determining where the boundary between vowel and consonant lies when the transition between them is gradual. This claim is tested in Persian (Farsi), which has length contrasts in all classes of consonants, including glides and laryngeals. Persian geminates were compared to singletons in three different speaking rates and seven different consonant classes. Geminates were found to have longer constriction intervals than singletons, and this length effect interacted with both speaking rate and manner of articulation. In one of two perception experiments, Persian speakers identified consonants as geminate or singleton in stimuli in which the constriction duration was systematically varied. The perceptual boundary between geminates and singletons was most sharply defined for obstruents and least so for laryngeals, as reflected by the breadth of the changeover region in the identification curve. In the other perception experiment, subjects identified the length class of glides differing in constriction duration and formant transition duration. Longer formant transitions led to more geminate responses and to a broader changeover interval.Persian (Farsi) has a contrast in consonant length, as illustrated by the minimal pair baenɑ 'building' -baennɑ 'builder'. This contrast extends to all the consonant classes in the language, listed in Table 1 (Samareh 1977(Samareh , 1985Mahootian 1997;Majidi & Ternes 1999;Deyhime 2000;Bijankhan & Nourbakhsh 2009;Hansen 2004Hansen , 2012. Samareh (1977) does not include geminate [ʒʒ] or [ɢɢ] in his inventory of Persian clusters, and we know of no instances of these geminates in Persian words. Yet we find that Persian speakers freely produce them in reading nonsense words with the relevant consonants marked with the length mark. This suggests that these are accidental gaps, and that these two consonants are not systematically avoided.With that caveat, the consonants in Table 1 all occur as singletons or as geminates, where the geminate differs from the corresponding singleton in having a longer constriction interval. The geminate has the same feature specification as the corresponding singleton, but