Several parts combine into one character.

階 is a phonetic-semantic compound: 阝 (the "mound / hill" radical, on the left side) + 皆 (jiē, "all / together" — providing the sound). The composite reads as "the orderly arrangement of steps cut into a hillside" = stairs. From this concrete physical scene grew the abstract sense "rank / class / stage / hierarchy" — social ladders, career stairs, and developmental phases all conceived as steps. Mainland simplified to 阶.

Korean reading "gye." 階段 (gyedan, stairs — physical staircase), 階級 (gyegeup, social class / military rank), 段階 (dangye, stage / phase), 上流階級 (sangnyu-gyegeup, upper class), 中産階級 (jungsan-gyegeup, middle class). Korean sociology and politics vocabulary leans heavily on 계 vocabulary; 계급 has Marxist political weight in Korean discourse.

Mandarin jiē, 1st tone (simplified 阶). 阶级 (jiējí, class — central term in modern Chinese political theory: 资产阶级 zīchǎn jiējí "bourgeoisie," 工人阶级 gōngrén jiējí "working class"), 阶段 (jiēduàn, stage / phase). For everyday "stairs," modern Mandarin uses the alternative 楼梯 (lóutī, "building-ladder"). 阶 in Mandarin tilts toward abstract / political register.

Japanese on-reading カイ (kai) — 階段 (kaidan, stairs — everyday word), 階級 (kaikyū, class / rank — used for military and social classes). Critically in Japanese, 階 functions as a counter for floors of a building: 一階 (ikkai, first floor), 二階 (nikai, second floor), 三階 (sankai, third floor), and so on. Every Japanese building uses 階 in its directory: ボタンの「3」を押すと3階に着きます ("press button 3 to reach the 3rd floor"). The kai counter is essential everyday vocabulary.

Memory aid: a hillside (阝) cut into orderly all-in-line (皆) terraces — physical steps became hierarchy and floor levels.