式

Outer 弋 — the indexing radical. Originally a pictograph of a stake or short retrieval-arrow with a cord attached, used as a fixed marker. The 'fixed reference point' meaning anchors 式 as 'standard, pattern, ceremony' — the established way of doing things. MMaH places 弋 as a discontinuous wrap (one stroke top, two strokes bottom-right) around the inner 工.

Inner 工 supplies the sound (gōng → shì shows heavy drift, but the pairing was established in Old Chinese). 工 itself pictures a carpenter's square or measuring tool. Combined with 弋 outside: a fixed measuring standard — exactly what a 式 (formula, pattern, ceremony) is. The inner workman's tool sets the pattern, the outer marker fixes the reference.