木 on the left is the tree radical, the pictograph of a tree with branches and roots. It marks 梯 as a wooden object, grouping it with 桌 (table), 椅 (chair), and 楼 (storey).
弟 supplies the reading, shifting from dì to tī along an old initial pair. It also adds a faint semantic: 弟 originally drew cord wound in successive turns down a stake, the same idea as the rungs of a ladder stepping down in order.
