～下
～か · ～ka · JLPT N2 · noun
under ~
下 is a line with a mark under it: ～下 (ka) = under ~, as in 支配下
The oldest form of 下 was a horizontal line with a short stroke below it, meaning below. As the suffix ～か it marks a condition you are under: 支配下 (shihaika) is under control, 影響下 (eikyōka) is under the influence.
下 — horizontal line 一 + mark below
下 is an indicative character: a long horizontal line represents a reference level, and a shorter stroke placed beneath it points to the area below. The original meaning 'below, down' comes directly from this spatial diagram, and it later extended to verbs like 'descend' and 'lower'.