Picture a line that starts at one point and keeps going round it, each loop sitting a little outside the last: a snail shell, a thumbprint, milk stirred into coffee. Every use of this word can be held in that one picture — sometimes the whole pattern, sometimes one ring of it, sometimes the thing that does the turning.
whorl 的畫面可以想成一條線從一個中心點出發，繞著它一圈一圈往外轉，每一圈都比上一圈大一點。蝸牛殼是這樣。指紋是這樣。奶精被湯匙攪進咖啡也是這樣。這個字的每一種用法，都可以放進這一個畫面裡：有時候講的是整片紋路，有時候是其中的一圈，有時候是那個負責轉的零件。
From the word’s history
The word is a changed spelling of whirl, the verb for turning fast — so a turning movement sits inside the word, and every sense is something that turns or the mark that turning leaves.
字源資料說 whorl 是 whirl（旋轉）變出來的寫法。可以想成：這個字裡本來就裝著一個「轉」的動作。後來它被用來指轉出來的痕跡，也被用來指會轉的零件。所以不管是花紋、是圈數、還是紡輪，底下都是同一個轉字。
Wiktionary, "whorl"
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