Flapping or tapping, also known as alveolar flapping, intervocalic flapping, or t-voicing, is a phonological process found in many varieties of English, especially North American, Ulster, Australian and New Zealand English, whereby the voiceless alveolar stop consonant phoneme /t/ is pronounced as a voiced alveolar flap [ɾ], a sound produced by briefly tapping the alveolar ridge with the tongue, when placed between vowels. In North American English, /d/, the voiced counterpart of /t/, in such positions is also frequently pronounced as a flap, making pairs of words like latter and ladder sound similar or identical. In similar positions, the combination /nt/ may be pronounced as a nasalized flap, making winter sound similar or identical to winner.

Flapping of /t/ is sometimes perceived as the replacement of /t/ with /d/; for example, the word butter pronounced with flapping may be heard as "budder".[1]

In other dialects of English, such as South African English, Scottish English, some Northern England English (like Scouse), and older varieties of Received Pronunciation, the flap is a variant of /r/ (see Pronunciation of English /r/).[2]

The terms flap and tap are often used synonymously, although some authors make a distinction between them. When the distinction is made, a flap involves a rapid backward and forward movement of the tongue tip, while a tap involves an upward and downward movement.[3] Linguists disagree on whether the sound produced in the present process is a flap or a tap, and by extension on whether the process is better called flapping or tapping,[4] while flapping has traditionally been more widely used.[5][6] Derrick & Gick (2011) identify four types of sounds produced in the process: alveolar tap, down-flap, up-flap, and postalveolar tap (found in autumn, Berta, otter, and murder, respectively).[7]

Flapping of /t/ and /d/ is a prominent feature of North American English. Some linguists consider it obligatory for most American dialects to flap /t/ between a stressed and an unstressed vowel.[6][8] Flapping of /t/ also occurs in Australian, New Zealand and (especially Northern) Irish English, and more infrequently or variably in South African English, Cockney, and Received Pronunciation.[9][10][11]

The exact conditions for flapping in North American English are unknown, although it is widely understood that it occurs in an alveolar stop, /t/ or /d/, when placed between two vowels, provided the second vowel is unstressed (as in butter, writing, wedding, loader).[5][12] Across word boundaries, however, it can occur between any two vowels, provided the second vowel begins a word (as in get over [ɡɛɾˈoʊvɚ]).[5][12] This extends to morphological boundaries within compound words (as in whatever [ˌwʌɾˈɛvɚ]).[13] In addition to vowels, segments that may precede the flap include /r/ (as in party)[8][14] and occasionally /l/ (as in faulty).[15][16] Flapping after /l/ is more common in Canadian English than in American English.[17] Syllabic /l/ may also follow the flap (as in bottle).[18] Flapping of /t/ before syllabic /n/ (as in button) is observed in Australian English,[19] while [t] (with nasal release) and [ʔ] (t-glottalization) are the only possibilities in North American English.[20]

Morpheme-internally, the vowel following the flap must not only be unstressed but also be a reduced one (namely /ə/, morpheme-final or prevocalic /i, oʊ/, or /ɪ/ preceding /ŋ/, /k/, etc.[a]),[22][23] so words like botox, retail, and latex are not flapped in spite of the primary stress on the first syllables,[8] while pity, motto, and Keating can be.[22] The second syllables in the former set of words can thus be considered as having secondary stress.[5]

Word-medial flapping is also prohibited in foot-initial positions. This prevents words such as militaristic, spirantization, and Mediterranean from flapping, despite capitalistic and alphabetization, for example, being flapped. This is known as the Withgott effect.[24][25]

In North American English, the cluster /nt/ (but not /nd/) in the same environment as flapped /t/ may be realized as a nasal flap [ɾ̃]. Intervocalic /n/ is also often realized as a nasal flap, so words like winter and winner can become homophonous.[26] According to Wells (1982), in the United States, Southerners tend to pronounce winter and winner identically, while Northerners, especially those from the east coast, tend to retain the distinction, pronouncing winter with [ɾ̃] or [nt] and winner with [n].[27]

Given these intricacies, it is difficult to formulate a phonological rule that accurately predicts flapping.[6] Nevertheless, Vaux (2000) postulates that it applies to alveolar stops:

Exceptions include the preposition/particle to and words derived from it, such as today, tonight, tomorrow, and together, wherein /t/ may be flapped when intervocalic (as in go to sleep [ˌɡoʊɾəˈslip]).[29] In Australian English, numerals thirteen, fourteen, and eighteen are often flapped despite the second vowel being stressed.[30][31] In a handful of words such as seventy, ninety, and carpenter, /nt/ is frequently pronounced as [nd], retaining /n/ and voicing /t/, although it may still become [ɾ̃] in rapid speech.[32][33]

Flapping is a specific type of lenition, specifically intervocalic weakening. It leads to the neutralization of the distinction between /t/ and /d/ in appropriate environments, a partial merger of the two phonemes, provided that both /t/ and /d/ are flapped.[4][34] Some speakers, however, flap only /t/ but not /d/.[35] For speakers with the merger, the following utterances sound the same or almost the same:

In accents characterized by Canadian raising, such words as riding and writing may be flapped yet still distinguished by the quality of the vowel: riding [ˈɹaɪɾɪŋ], writing [ˈɹʌɪɾɪŋ].[36] Vowel duration may also be different, with a longer vowel before /d/ than before /t/, due to pre-fortis clipping.[37]

The origins of the T-to-R rule lie in the flapping of /t/ and the subsequent reinterpretation of the flap as /r/, which was then followed by the use of the prevailing variant of /r/, namely the approximant [ɹ]. It is applied in Northern England English and it is always stigmatized. The application of that rule means that shut in the phrasal verb to shut up /ʃʊrˈʊp/ has a different phonemic form than the citation form of the verb to shut /ʃʊt/. The rule is typically not applied in the word-internal position.[38]

The T-to-R rule has also been reported to occur in the Cardiff dialect (where the merged consonant can surface as either an approximant or a flap) and South African English (where only a flap is possible). In the Cardiff dialect, the rule is typically applied between any vowel (including long vowels) and /ə/ or the reduced /ɪ/ (also across word boundaries), so that starting /ˈstaːtɪŋ/ and starring /ˈstaːrɪŋ/ can be homophonous as [ˈstaːɹɪn ~ ˈstaːɾɪn]. In South African English, the merger is possible only for those speakers who use the flapped allophone of /r/ (making the startingstarring minimal pair homophonous as [ˈstɑːɾɪŋ]), otherwise the sounds are distinguished as a flap (or a voiceless stop) for /t/ ([ˈstɑːɾɪŋ ~ stɑːtɪŋ]) vs. approximant for /r/ ([ˈstɑːɹɪŋ]). There, the merger occurs word-internally between vowels in those environments where flapping is possible in North American English.[39][40]