Lower 木 is the tree radical and the entry's filing point. The character pictures a tree whose topmost branches are still tender saplings - a tree not yet at full growth - which gives 未 its meaning of not yet, the eighth earthly branch.
Top horizontal is a short branch-line sitting above the tree below. As an indicator it marks the youngest top branches that have not yet grown out fully - the not-yet sense at the core of 未, signalling immaturity rather than ripeness.
