Phonosemantic compound. 阝 represents the meaning and 介 represents the sound. Simplified form of 階.

Evolution

Oracle script

(~1250-1000 BC)

Bronze script

Early Warring States (~400 BC)

Seal script

Shuowen (~100 AD)

Clerical script

Eastern Han dynasty (25-220 AD)

Traditional script

Modern

Simplified script

Modern

 More