Lev Vygotsky described the zone of proximal development (ZPD) as a space between what a person knows (an action that can be performed easily) and what a person doesn't know (what is considered difficult).[30] Vygotsky theorised that learning is most effective within this zone.[31] Coaches facilitate effective learning by providing coachees with activities within the ZPD, which are neither too easy nor too challenging (this is a process called scaffolding).[30][31]