兩邊對得上
拿掉寫信的人，只留下「彼此對得上」這層關係。字根 com-（彼此）+ respondere（回應）本來講的就是兩個東西互相對應，所以這個用法不是從書信引申出來的，而是跟書信並排的另一種用法——只是這次互相回應的是兩件事、兩組數字。
a matching relationship · noun · B2
A connection or matching relationship between two or more ideas or facts.
對應；相符；關聯
核心畫面
Two sides answering each other. Picture letters going back and forth between two people over months: your letter goes out, theirs comes back, and each one answers the one before it. The word does not name a single letter — it names the two-way fit between them. Once you see it as 'these two answer each other', it stretches to things that are not people at all.
可以想成「兩邊互相回應、彼此對得上」。畫面是兩個人幾個月來一封來一封往的信：你寄出去一封，對方回一封，每一封都在回應…更多收起
可以想成「兩邊互相回應、彼此對得上」。畫面是兩個人幾個月來一封來一封往的信：你寄出去一封，對方回一封，每一封都在回應前一封。這個字指的不是單獨一封信，而是那個一來一往、彼此接得上的關係。一旦看成「這兩邊互相對得上」，它就不只能用在人身上了。From the word’s history
From Medieval Latin correspondere — com- 'together, with each other' + respondere 'to answer', originally used of two things in mutual action. Etymonline dates 'to be in agreement, in harmony with' to the 1520s and the narrower 'communicate by means of letters' to the 1640s. That is why one word covers both letters and matching: writing back and forth is one special case of two sides answering each other.
來自中世紀拉丁文 correspondere——com-「一起、彼此」+ respondere「回應」，原本就是用來講「兩個東西互相作用」。Etymonline 記載「相符、一致」這個意思出現在 1520 年代，比較窄的「用書信往來」則到 1640 年代才有。這解釋了為什麼「書信」和「對應」會是同一個字：通信只是「兩邊互相回應」這個關係裡的一種特例，而不是後來硬借來的意思。
Online Etymology Dictionary (etymonline.com), entries 'correspond' and 'correspondence'.
From the picture
Also in this word
exchanging letters · noun · B1
The process of sending letters to someone and getting letters back from them.
通信；書信往來
letters exchanged · noun · B1
Letters going back and forth between two sides, mostly in business or official dealings.
信件；往來書信
原始詞典資料8 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
correspondencenoun/ˌkɒɹ.ɪˈspɒn.dəns/
- 1
obsoleteMutual communication or discourse:
- 2
Mutual communication or discourse:
- 3
Mutual communication or discourse:
- 4
Mutual communication or discourse:
- I'm doing time in Oklahoma and really would appreciate some correspondence.
- 5
Congruity or similarity between different things, people, etc
- For, if praxis signifes the whole of man's part in history, the value of intellectual production as an aspect of that whole is to be measured by the mind's ability to 'express' changing historical situations, and not by the correspondence between some 'objective' universe and the description of it.
- 6
Congruity or similarity between different things, people, etc:
- 7
mathematicssciencesset-theoryCongruity or similarity between different things, people, etc:
- 8
lifestylereligiontheologyCongruity or similarity between different things, people, etc:
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。