凳

Bottom 几 — pictograph of a low table or armrest stand, the indexing radical for 凳. It contributes the physical 'piece of furniture' meaning directly: 几 names short-legged stools and side tables. Stack 'climbing' on top of 'stand' and you get exactly a stool — the thing you mount to sit on.

Top 登 supplies sound and meaning. Sound: dēng → dèng, just a tone change. Meaning: 登 originally pictured two feet stepping onto a sacrificial vessel — the act of climbing up. A stool is exactly 'the thing you climb onto,' the low platform that raises a seated body off the floor. Same phonetic in 蹬 (stamp) and 噔 (clatter).