Read the whole elevation
A sash or casement belongs to the façade around it. Meeting rails, mullions and glazing bars should align with the structure and with neighbouring openings.
Thickness changes character
Frames that become too heavy reduce glass area and flatten the architecture. Good sections balance strength, weathering and the visual lightness expected of the building.
Details work together
Cills, architraves, horns, hardware and paint all affect what the eye reads. Resolving them together is more valuable than perfecting one detail in isolation.