Amiens Cathedral, upper choir, north

Our view is partially blocked by the trees around the bishop's palace.
Compare this view with the upper nave: how many differences can you spot?
--the flying buttresses are no longer solid stone props but have delicate vertical panels like window tracery. Note that this begins already in the transept.
--the upper wall is no longer horizontal but is broken by gables.
--the window tracery in the upper choir clerestory is more complex than the nave.

How do you explain these differences?
--the choir is occupied by the clergy; the nave by layfolk
--the choir provides the theater for the Eucharistic sacrament
--the upper choir was done by a different architect: Master Renaud de Cormont, 1250s-60s. The upper nave by his father, Thomas de Cormont, 1230s.