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 by adding a stroke to ㅅ (s, “s”), which itself is a visual representation of the incisor, because both /s/ and /t͡s/ are sibilants ("incisor sounds" in Sejong's terminology).
Gari Ledyard proposes that Sejong derived both ㅅ and ㅈ from the 'Phags-pa letterꡛ(s). Ledyard connects the unusual case of a single 'Phags-pa letter giving rise to two different hangul glyphs to the fact that Sejong appears to have used only the 'Phags-pa letters for voiceless non-aspirated obstruents; Korean has two such sibilants, the fricative/s/ and the affricate/t͡s~t͡ɕ/. 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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