四环素

• 囗(frame; surround)RADICAL
• 儿(person; foot)ASSOCIATIVE

Four sides of a square frame surround two legs — the ancient form of the number four.

• 王(jade)RADICAL
• 不(empty; void; sky)PHONETIC

A ring made from precious jade has a circular shape with no beginning or end.

• 糸(silk thread; filament)RADICAL
• 龶(coarse undyed thread bundle; top part of thread)ASSOCIATIVE

Raw silk that hasn't been dyed suggests the meaning of pure, simple, and basic.