A thin flat sheet with shapes cut out of it, pressed against a surface. Colour can only get through the holes, so the sheet decides the edges and your hand does not. Lift it, and the shape is there — the same shape, as many times as you like.
stencil 的畫面可以想成一張挖了洞的薄片，壓在一個表面上。顏料只能從洞裡下去，其他地方被擋住。所以形狀的邊界是板子決定的，不是手決定的。把板子拿開，圖就留在那裡了。想印幾次就印幾次，每一次都長得一樣。這個字的三個用法，就分別站在這個畫面的三個位置：那塊板子、那個動作、還有留下來的那個圖。
From the word’s history
The supplied source traces the noun to Middle English stencellen 'to garnish with bright hues', from Old French estenceler 'to spark', ultimately Latin scintilla 'spark'. The oldest idea is putting bright colour onto something; the cut-out sheet came later and narrowed it.
packet 裡的來源說這個名詞很可能出自中古英語 stencellen「用鮮亮的顏色裝飾」，再往前是古法文 estenceler「使閃爍」，最早是拉丁文 scintilla「火花」。所以最原始的想法是「把亮色點上去」，鏤空板子是後來才把這個意思收窄的。來源同時指出，動詞是從名詞長出來的。
Wiktionary, "stencil"
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