当且仅当

• 口(mouth; sound)RADICAL
• 當(equivalent; opportune)PHONETIC

The sound of a bell ringing from an opening makes a "clanging" noise that rings out at just the right moment.

• 且(altar; pedestal; stand)ASSOCIATIVE

A stacked altar table shape represents something temporary or just added on.

• 亻(person; human; people)RADICAL
• 又(again; more)PHONETIC

A person alone, no more no less, just like the doubling symbol reminds us: that's all.