平的、左右走向的（形容詞）
這是核心畫面本身，用來形容東西的方向。
flat and level · adjective · C1
Flat and level, going from side to side like the ground, not up and down.
水平的
核心畫面
A way to hold it: think of the flat line where sea meets sky, then bring that direction indoors. Anything horizontal runs the same way — left to right, flat and level, never up and down.
先看遠方海天相接那條線，它是平的、往左右延伸——horizontal 就是「跟那條線同一個方向」。把這個方向搬進日常…更多收起
先看遠方海天相接那條線，它是平的、往左右延伸——horizontal 就是「跟那條線同一個方向」。把這個方向搬進日常：桌面、地板、平躺的身體、圖表的橫軸，全都是這個走向。它永遠是相對於 vertical（上下）來定位的，兩個字合起來把空間切成兩個最基本的方向。From the word’s history
It comes straight off horizon (Latin horizōn, stem horizont-): horizontal literally means "of the horizon." The word was built by pointing at that flat far line and saying "that direction."
這個字直接長在 horizon 上（拉丁 horizōn，詞幹 horizont-），字面就是「地平線的／屬於地平線的」。也就是說，人類當初要指認「水平」這個方向時，用的辦法就是指著遠方那條天地交界線說「這個方向」。所以核心畫面和字源完全是同一件事——記住地平線，就記住了 horizontal。
Wiktionary, entry "horizontal" (< Middle French horizontal < horizon, Latin horizōn, stem horizont-)
Two branches
那條線、那個姿勢（名詞）
同一個方向，從「形容詞：東西是這個走向」變成「名詞：這個走向本身」——把方向當成一個可以指認的東西來講。
a horizontal line · noun · C1
A line or a surface that is flat and goes from side to side, not up and down.
水平線
原始詞典資料10 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
horizontaladj/ˌhɒɹɪˈzɒn.təl/
- 1
Perpendicular to the vertical; parallel to the plane of the horizon; level, flat.
- horizontal lines
- I also claim the combination of the separator shaft with the shoe, and with the pitman, for the purpose of imparting to it a recriprocating horizontal movement, and a vibratory movement on its axis at the same time,...
- 2
businessmarketingRelating to horizontal markets
- 3
archaicPertaining to the horizon.
- As when the Sun new ris'n / Looks through the Horizontal misty Air
- 4
beveragesfoodlifestyleInvolving wines of the same vintages but from different wineries.
- 5
entertainmentlifestylemusicHaving the two notes sound successively.
- 6
lifestylesexsexualityRelating to sexual intercourse.
- horizontal tango
- Comparing a man who inspires you intellectually and makes you laugh with a guy who fulfils all your horizontal desires means you’re not comparing like with like.
- 7
biologynatural-sciencesBeing or relating to the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship.
- Infectious agents may spread by horizontal transmission.
horizontalnoun/ˌhɒɹɪˈzɒn.təl/
- 1
A horizontal component of a structure.
- 2
geographygeologynatural-sciencesHorizon.
- 3
A Tasmanian shrub or small tree whose main trunk tends to lean over and grow horizontally, Anodopetalum biglandulosum
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。