繞一圈的帶子
這一組就是核心畫面本身：一條看得到、摸得到、扣得緊的帶子。
noun · C1
A strong band, usually of leather, that passes under a horse's belly and is pulled tight to hold a saddle or a load firmly in place.
肚帶
核心畫面
Hold one picture: a band going all the way around a middle and meeting itself. On a horse it is a real leather strap under the belly, pulled tight so the saddle cannot slide. Take the strap off, lay it flat and measure it, and you have the other meaning - how far it is around a tree, a pipe or a waist. Even the maths use counts the smallest way around a loop.
girth 可以想成一條繞過中間、然後接回自己的帶子。在馬身上，它是真的一條皮帶，穿過肚子底下拉緊，馬鞍才不會滑掉。…更多收起
girth 可以想成一條繞過中間、然後接回自己的帶子。在馬身上，它是真的一條皮帶，穿過肚子底下拉緊，馬鞍才不會滑掉。把那條帶子解下來、攤平、量一量，就成了另一個意思：樹幹、水管或腰繞一圈有多長。連數學裡的用法也在數同一件事——繞最小的一圈要走多遠。From the word’s history
The packet's source traces girth to Middle English girth, from Old Norse gjord, from a Proto-Indo-European root meaning to encircle, enclose, belt. It names gird and German Gurt as relatives. So the encircling band is the oldest recorded idea in the word.
這份資料的來源說，girth 來自中古英語 girth，再往前是古諾斯語 gjǫrð，最早的字根意思是圍起來、圈住、束帶。來源還列出 gird 和德語 Gurt 這些同源字。所以繞一圈的帶子這個概念，在這個字裡是有紀錄的最早一層。
Wiktionary 'girth' etymology as supplied in this packet (source_ref kaikki:3f6f69a190036ae4:524088070, 524112045)
Two branches
繞一圈有多長
把那條帶子解下來攤平，看的就不再是帶子本身，而是它的長度。繞一圈的東西變成繞一圈的數字。
noun · B2
The distance all the way round something thick, such as a tree, a pipe or a person's body at the middle. It tells you how big something is around, not how tall or how long it is, and it is used most about things that are large.
周長／腰圍