第

Bamboo radical on top — also 第's indexing radical (Kangxi #118 in its compressed top form). Original sense: ranking marks notched on bamboo strips, the way ancient documents and lists were tallied. From bamboo-tally came 'order, rank, sequence' — 第一 (first), 第二 (second). Same radical in 笔 (brush), 篇 (chapter), 节 (joint), 等 (rank).

Bottom 5 strokes — graphically a contracted form of 弟 (standalone 7 strokes). The middle horizontal and the descending bow have fused, pulling the standalone shape from 7 down to 5 in this position; this is a regular bamboo-radical contraction. 弟 supplies the sound directly (dì → dì, no shift) and adds a faint semantic link — younger brothers are ranked by birth order, the same ranking sense 第 carries.