回禮與回應：人跟人之間的一來一往
這一族把「來回」放到兩個人之間：一方先送出，另一方再把同樣性質的東西送回去。差別只在送回去的是行為，還是心裡的感覺。
verb · C1
To do or give something back to someone because they have done or given something similar to you.
回報；禮尚往來
核心畫面
You can picture reciprocate as one line with movement running along it both ways. Something goes out, and then the same kind of thing comes back along the same line. A bag of oranges crosses the hallway, and a tray of egg tarts crosses back. A feeling goes towards a person, and the same feeling returns. Inside a machine, nothing crosses at all: the part itself goes out and comes back, out and back, for as long as the engine runs.
reciprocate 可以想成一條來回的線。東西先從這邊過去，再從那邊回來。鄰居送橘子過來，我們烤蛋塔送回去。心意…更多收起
reciprocate 可以想成一條來回的線。東西先從這邊過去，再從那邊回來。鄰居送橘子過來，我們烤蛋塔送回去。心意過去，同樣的心意回來。到了機器裡，就不是兩個人之間的來回了，而是同一個零件沿著同一條線出去又回來，一直重複。抓住這個「一來一往」，三個意思就串得起來。From the word’s history
Latin recīprocō meant 'to move back and forth', from reciprocus 'back and forth', possibly from a phrase like reque proque, where re- is 'back' and prō is 'forwards'.
拉丁文 recīprocō 的意思就是「來回移動」，來自 reciprocus（來回的）；再往前可能是 reque proque 這樣的說法，re- 是往回，prō 是往前。所以機械上的「往復」不是後來才長出來的用法，反而最貼近字根原本的畫面，而人跟人之間的「回報」是同一個畫面搬到人身上。
Wiktionary entry for “reciprocate”, etymology text supplied in this batch's evidence packet (source_ref kaikki:3f6f69a190036ae4:1175570597:16905)
Two branches
機械的往復：零件沿同一條線來回
這一族沒有第二個人，只剩下那條來回的線本身。它其實離拉丁字根最近：recīprocō 本來講的就是這種來回移動。
verb · C1
Of a part of a machine: to move backwards and forwards in a straight line, again and again.
往復運動