Complex sentence: There is therefore an inherent reason for paintings to be flat and rectangular-although artists have often departed from the norm, especially in circumstances requiring special commissions, an example being the paintings Henri Matisse created for Albert C. Barnes and for Nelson Rockefeller.
Split into simpler sentences: There is therefore an inherent reason for paintings to be flat and rectangular. Although artists have often departed from the norm, especially in circumstances requiring special commissions, an example being the paintings Henri Matisse created for Albert C. Barnes and for Nelson Rockefeller.