One way slabs are designed to transfer their loads to only two opposite support walls. Generally for spans up to 3.60 meters and with length/breadth ratio is greater than 0.60 one way slabs can be provided.the reinforcement will be along the longer direction.

Two way slabs are designed to transfer their loads to all the four support walls. Generally for spans greater than 3.60 meters and with length/breadth ratio is less than 0.60 two way slabs can be provided.the reinforcements will be placed along the length and the breadth i.e. the reinforcements will be perpendicular to one another.

A one way slab, have it structural strength in shortest direction.

A two way slab, have it structural strength in in two direction.