Paper accessible at http://www.mksjournal.org/ International audience The paper presents a historical phonological explanation for the Bangkok Thai khw-f alternations as in faa A1 for Standard Thai khwaa A1 'right (side)'. While this phenomenon has attracted the attention of scholars in various linguistic fields such as the phonetics of Thai, sociolinguistics of Bangkok Thai and applied linguistics (teaching Thai as a "foreign" language), the potential of Tai dialectology and historical linguistics has not been fully taken into account. The paper argues that such alternations are widely attested among Tai varieties, that they are neither a synchronic alternation, nor an innovation in Bangkok Thai, and that the phenomenon ultimately goes back to a change that turned PSWT *xw- into /f-/.