Jacob’s ladder in the context of DFT can be seen as a kind of ‘roadmap’ to find approximations of the exchange-correlation functional that get us closer to the ‘divine’ exchange-correlation functional, one step (or rung) at a time. The first rung is the local density approximation. Built on this (= the next rung) are the generalized gradient approximations. The third rung are the meta-GGAs. At the fourth rung, exchange is treated exactly and at the last rung, a part of the correlation is evaluated exactly. Every step is on top of the others, so this is why we say it is a ladder. If the complete correlation can be treated exactly, the divine functional would have been found.