仄声

In Classical Chinese phonology, the four tones split into two prosodic classes: 平 (píng, 'level') and 仄 (zè, 'oblique', covering rising 上, departing 去, and entering 入 tones). The level/oblique alternation is the backbone of regulated verse (律诗 lǜshī) and parallel couplets (对联 duìlián).

memory

仄 (zè) literally means 'tilted / slanting' — and the oblique tones are precisely the ones that slant up or down rather than holding level.