根本
こんぽん · konpon · JLPT N1 · noun
foundation
根 — tree radical 木 + 艮 (phonetic)
根 is a semantic-phonetic compound. The left side 木 ('tree') gives the meaning, since the character originally referred to the roots of a tree. The right side 艮 supplies the sound. From 'tree root' it extended to mean 'root, base, foundation' in general.
本 — tree 木 + mark at base
本 is a pictograph of a tree (木) with a stroke at its base, marking the roots. From this 'root' meaning it extended to 'origin,' 'source,' and 'foundation.' In modern usage it also serves as a measure word for bound objects like books.