Amiens Cathedral, north transept

The north transept facade is dominated by a huge circular rose window. The stained glass is held up by a complex pattern of thin sticks of stone known as tracery. Much can be learned from a comparative analysis of tracery patterns which appear to "develop" or "evolve" over time Here we see a design centered upon a five-pointed star from which 15 petals are developed. The petals and panels are filled with geometric shapes, mostly trefoils. You can see that the window has been propped up with the insertion of two little buttresses that support the upper rim. And the gable of the transept roof is made of wood, not stone, in order to reduce weight.
This is an area of structural distress in the building--the complex tracery patterns of the rose belong to the mid-14th century well after the cathedral was "finished"