Which development took place within the northern international movement of the 15th century

The oil painting and the impressive attention to detail.

Explanation:

Jan van Eyck is by far the best-known artist of this period. He introduced the oil painting technique, which brought a new aesthetics for art. Its main feature is the chromatic mix and gloss giving exceptional finish, as well as flexibility due to the slow drying of the paint, allowing the artist to change his work when necessary.

i believe their role is to instruct certain sections on when they are to join and when they are to stop