to have to; must; compulsory; necessarily
必 (bì) · 5 strokes
Pictograph of a weapon handle, now written as 柲. 八 was later added as a decorative mark. Similar to 戈 but without the blade. The current meaning is a phonetic loan.
须 (xū) · 9 strokes
Pictograph of a person with a bearded face. Simplified form of 須. The meaning "must" is a phonetic loan.