本文在歷史比較法與歷史層次分析法的基礎上，特別著眼於歷史層次對應的系統性研究，我們認為歷史層次分析不能僅限於單一方言或單一韻攝的研究，應該進一步從音韻分合關係切入檢視歷史層次，方能確實掌握閩方言複雜多重的層次脈絡。在此觀點下，我們觀察分析閩方言2、4等同讀的歷史層次，首先透過跨方言點、跨韻攝等第的比較研究，在複雜的內部音變與層次競爭所造成的層次不等現象中，確實離析這項歷史層次的方言對應關係;然後從方言對應關係中，構擬原始韻讀以說明各方言點的音變情形;最後，藉自2、4等同讀層次即是表現3、4等有別的音韻系統特色，判定其反映的歷史時間應屬六朝時期，不過，閩方言內部也有自身的創新音變，使得2等韻與4等韻同讀。

This article focuses on a phonological correspondence in a historical stratum of Min, in which the words from Grade II and Grade IV of traditional Chinese phonology have identical finals. First, we distinguish this historical stratum in Min by using the methods of historical comparison and internal reconstruction. Then we reconstruct the original finals and explain the probable sound changes in dialects of Min. Last, we determine that this situation of identical finals arose in the Old Jiang-dong dialect of the Six-Dynasties Period, but that Min separately carried out a sound innovation which resulted in its Grade II and Grade IV finals being identical.