韻圖都常常遷就古代嘅反切與古代嘅分類——即韻圖多係用來分析經典韻書嘅，而不是當時語音嘅分析，後代讀書人的發音要是同韻圖作者不同，便很難瞭解韻圖代表嘅發音。最後導致韻圖嘅作者與其他學者必須再編寫韻圖嘅「門法」，來對韻圖做解釋。隨著時間推移，語音嘅演變，門法就越寫越多，最後到了明清時代，韻圖附嘅門法可以多到十幾條，不僅沒有使韻圖變簡單，反而搞到韻圖成為「天書」。
Rhyme diagrams are also often accommodated to ancient anti-tangential and ancient classifications - that is, rhyme diagrams are mostly used to analyze classic rhyme books, rather than the analysis of the phonetics of the time, and if the pronunciation of later generations of readers is different from that of the author of the rhyme diagram, it is difficult to understand the pronunciation represented by the rhyme diagram. In the end, the author of the rhyme diagram and other scholars had to write the "door method" of the rhyme diagram to explain the rhyme diagram. With the passage of time, the evolution of phonetics, the more the door method was written, and finally in the Ming and Qing dynasties, the door method attached to the rhyme map could be more than a dozen, not only did not make the rhyme diagram simpler, but made the rhyme map become a "book from heaven".