The Hunmin Jeongeum Haerye, the treatise introducing the principles behind the Korean alphabet written by its inventor King Sejong in 1446, explains that this glyph was derived from the "outline of the tongue" when pronouncing this "semi-lingual sound" (半舌音), but does not elaborate.
Gari Ledyard suggests that Sejong may have had his inspiration for ㄹ from the 'Phags-pa letterꡙ(l), although he is more tentative here than with the Hangul letters for the plosives. Ledyard gives evidence that Sejong was inspired by 'Phags-pa for the basic glyph forms, although he changed the shapes of the letters drastically in order to enhance the simplicity and rationality of his script, and the ultimate shape of the letters may indeed have been influenced by that of the speech organs (Ledyard 1997).
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