简

Top bamboo radical (Kangxi #118 竹 in its 6-stroke compressed top form). Anchors the original meaning: 简 originally referred to bamboo strips used as writing slips before paper. From the simple, brief nature of these slips came the modern senses 'simple, brief, succinct.' Same radical groups 篮 (basket), 笔 (brush), 答 (reply), 算 (calculate).

Bottom phonetic 间 - supplies the sound, shifting tone to jiǎn. The simplified 简 replaced traditional 簡's same-shape 間 phonetic with its modern simplified form 间. Standalone 间 means 'between, interval' - purely phonetic in 简, no semantic carryover.