In the previous post we saw Merge is SL-2 over dependency trees, and Move is TSL-2. For every movement feature f we project a separate tier that contains only lexical items that have a licensor feature f+ or a licensee feature f-. A tier is well-formed iff every lexical item with a licensee feature has a mother with a licensor feature, and every lexical item with a licensor feature has exactly one lexical item among its daughters that carries a licensee feature. It’s a pretty simple system. Despite that simplicity, it predicts a fundamental aspect of movement: island effects!