刀身本身：兩邊都開刃
這一支就是畫面本身，沒有轉彎。
adjective · B2
Of a knife, blade, or similar tool: sharp along both of its long sides, so it can cut whichever way it is moved.
雙刃的
核心畫面
One way to hold this word: a blade that has been sharpened on both of its long sides. There is no safe edge on it. The swing that cuts can also turn and cut the hand holding it, and that single picture is doing all the work in both uses of the word.
two-edged 可以想成一把兩邊都磨利的刀。這把刀身上沒有一條安全的邊。揮出去可以切到別人，一轉手也可能割到自己…更多收起
two-edged 可以想成一把兩邊都磨利的刀。這把刀身上沒有一條安全的邊。揮出去可以切到別人，一轉手也可能割到自己。整個字的兩種用法，都是從這一個畫面長出來的：一個講刀本身，一個講「同一個動作，兩個相反的結果」。From the word’s history
The word is simply two + edged, so the spelling itself shows the picture: a thing with two cutting edges.
這個字就是 two 加上 edged，字面上寫的就是那個畫面：兩條邊都是刃。所以比喻用法不必另外記，看得懂字面就看得懂比喻。
Wiktionary entry for two-edged: "From two + edged." (supplied in the evidence packet)
Two branches
往外也往內：一體兩面
把刀換成一件事，把「兩個刃」換成「兩個相反的結果」，畫面就搬到生活裡了。
adjective · B2
Having two sides to it, one good and one bad. A two-edged thing helps you and hurts you at the same time, and a two-edged remark praises and criticises in the same breath.
一體兩面的；好壞參半的