高度到膝蓋那條線
這一支就是照字面量高度，形容詞和名詞用的是同一條線。
adjective · B1
Tall enough to reach a person's knees. It is used about things that stand up from the ground, such as grass, plants, snow or a low fence, and about boots and socks that come up to the knee.
及膝的
核心畫面
Stand up straight and put your hand on your knee. That line — about half a metre off the ground — is what this word measures to. Grass can grow up to it, snow can pile up to it, a boot or a sock can be cut off at it, and a very small child can be no taller than it. The ruler is always a standing person's body, never a number.
可以想成你站直了，手放在膝蓋上的那一條線，離地大概半公尺。這個字量的就是那條線。草可以長到那裡，雪可以堆到那裡，靴子…更多收起
可以想成你站直了，手放在膝蓋上的那一條線，離地大概半公尺。這個字量的就是那條線。草可以長到那裡，雪可以堆到那裡，靴子和襪子可以在那裡收邊，而一個很小的孩子可以整個人還不到那裡。有意思的是，這把尺永遠是「一個站著的人」，不是公分數字——所以同樣一片草，對大人和對小孩說出來的畫面並不一樣。From the word’s history
Wiktionary records the word as simply knee + high (adjective): the two parts are doing exactly what they look like they are doing.
Wiktionary 記的組成就是 knee（膝蓋）＋ high（高的）：兩個零件做的正好就是表面上看到的事——高度到膝蓋。也因為這麼直接，英文很自然地拿同一個模子造出 ankle-high、waist-high、chest-high 一整串。
Wiktionary entry for knee-high (packet evidence kaikki:3f6f69a190036ae4:1267215065): "From knee + high (adjective)."
Two branches
小到只有膝蓋那麼高
把那條線反過來用：不是東西長到膝蓋，而是整個人還不到膝蓋。量的還是同一條線，只是被量的變成了人。
adjective · B2
Used when talking about the past to say that someone was a very small child then. The joke is that the person was only as tall as a grasshopper's knee.
還是小不點的時候