Left white radical (Kangxi #106) — originally a thumbnail or grain of rice, the prototype of pale and plain. Indexes
的 alongside
百 (hundred),
皇 (emperor),
皆 (all). The semantic link to the modern grammar particle is faint:
的 was hijacked from the older 'bright / target / aim' senses for purely phonetic reasons, but the white-radical home address is preserved for lookup.