Explanation of the medial sounds中聲解
中聲者，居字韻之中，合初終而成音如呑字中聲是ㆍ，ㆍ居ㅌㄴ之間而爲ᄐᆞᆫ。卽字中聲是ㅡ，ㅡ居ㅈㄱ之間而爲즉。侵字中聲是ㅣ，ㅣ居ㅊㅁ之間而爲침之類。洪覃君業欲穰戌彆，皆倣此。
The medial lies at the middle of the sound of a letter and joins with the initial and the final to form the sound. The medial of the character 呑 tan is ㆍ; ㆍ stands between ㅌ and ㄴ to make ᄐᆞᆫ. The medial of 卽 jeuk is ㅡ; ㅡ stands between ㅈ and ㄱ to make 즉. The medial of 侵 chim is ㅣ; ㅣ stands between ㅊ and ㅁ to make 침. And so on in like manner. 洪 hong, 覃 dam, 君 gun, 業 eop, 欲 yok, 穰 yang, 戌 sul and 彆 byeol all follow this same pattern.
二字合用者，ㅗ與ㅏ同出於ㆍ，故合而爲ㅘ。ㅛ與ㅑ又同出於ㅣ，故合而爲ㆇ。ㅜ與ㅓ同出於ㅡ，故合而爲ㅝ。ㅠ與ㅕ又同出於ㅣ，故合而爲ᆑ。以其同出而爲類，故相合而不悖也。一字中聲之與ㅣ相合者十，ㆎㅢㅚㅐㅟㅔㆉㅒㆌㅖ是也。二字中聲之與ㅣ相合者四，ㅙㅞㆈㆋ是也。ㅣ於深淺闔闢之聲，並能相隨者，以其舌展聲淺而便於開口也。亦可見人之參贊開物而無所不通也。訣曰
The joining of two letters is as follows. ㅗ and ㅏ both came forth from ㆍ, and so they join to make ㅘ; ㅛ and ㅑ again both came forth from ㅣ, and so they join to make ㆇ; ㅜ and ㅓ both came forth from ㅡ, and so they join to make ㅝ; ㅠ and ㅕ again both came forth from ㅣ, and so they join to make ᆑ. Having come forth from the same place they are of one kind, and so even when joined together they do not clash.
Of the single-letter medials, ten join with ㅣ, namely ㆎㅢㅚㅐㅟㅔㆉㅒㆌㅖ; and of the two-letter medials, four join with ㅣ, namely ㅙㅞㆈㆋ. That ㅣ is able to follow after every sound, deep or shallow, closed or open, is because the tongue is spread flat and the sound is shallow, so that the mouth opens with ease. In this too one may see how man takes part in the opening out and completing of all things, and there is nowhere he does not reach. Let us sing it in brief.
母字之音各有中 須就中聲尋闢闔 洪覃自呑可合用 君業出則亦可合 欲之與穰戌與彆 各有所從義可推 侵之爲用最居多 於十四聲徧相隨
Every sound of the letter-mothers has its medial, so go to the medial and seek out its opening and closing there. hong and dam came forth from tan and so can be joined in writing, and gun and eop too are of one place and so can likewise be joined. yok and yang, and sul and byeol — each has that which it follows, and the sense may be grasped by inference. The use of chim is the greatest of all, for it follows after fourteen sounds in all.