3 main parts
The main subject: the Stable & the Holy Family
The landscape: the foreground (the nativity and the little people carrying offerings), and the background (small mountains & streams, villages, sheep-pen & paths)
Since the time of the earliest cribs of Marseilles, the nativity is often placed on the left or on the right of the composition in order to leave a large area for the landscape and to follow the progress of the little people walking on towards the stable clearly.
Background: it finishes and outlines the crib (a sky painted on a canvas, a painting representing a perspective)