本文以CMARC3及MARC 21的變長欄為例，介紹兩種MARC在變長欄對照中較為重要的概念與內容。首先羅列CMARC3變長欄常用欄位及分欄與MARC 21的對照，俾供圖書館及編目員進行快捷參考。接著說明CMARC3變長欄中特有的連接款目段 (4XX) 與MARC 21之欄位與指標轉換通則及其分欄內容 (尤其是$1內容) 的處理原則。隨後釐清在變長欄內才會使用到的標點符號及在兩種MARC變長欄轉換時，分欄可重複之欄位組配關係的處理方式。文末並針對連接款目段、標點符號與欄位組配上兩種MARC間的差異及特色，加以綜合出結論與建議，或可提供國內未來因應全媒體時代修訂相關規範的參考。

This paper compares the use of variable length fields in CMARC3 and MARC 21, introducing important related concepts and content. First, the most common variable length fields and subfields of CMARC3 and MARC 21 are listed in a table for comparison, to serve as a quick reference for libraries and cataloguers. Then specific linking sections (4XX) of CMARC3 variable length fields are mapped in tables to MARC 21 fields and indexes, in order to explain the principles for handling the conversion of subfield content (especially the $1 content) between the two formats. Then the use of the punctuation in variable length fields is clarified as well as solutions to deal with possible groupings of subfield-repeatable variable length fields occurring while converting between these two MARCs. Finally, this study compares the characteristics and differences between the two MARCs in terms of their linking fields, punctuation usage, and field groups, and offers conclusions and recommendations to serve as a reference for the future revision of related guidelines.