The stone decorations and patterns in the windows of Gothic cathedrals are not only spectacular, but also quite informational. Those patterns are called “tracery,” and their shape can tell you when they were created.
If the pattern is based on straight lines dividing the window, like the wheel of a bicycle, then you have Rayonnant Gothic, which means it’s probably from the 1250 to 1350 period.
The most striking example of Rayonnant Gothic is perhaps the rose window of the Strasbourg Cathedral:
Rose window, Strasbourg Cathedral, 13th century
But you can also see it in Paris and Chartres:
Rose window, Notre Dame de Paris, c.1250
Rose window, Chartres Cathedral, 13th century
From the outside, the Rayonnant south rose window of Notre Dame de Paris looks like this:
If, however, the pattern is based on S-shaped lines that look like flames, then you have Flamboyant Gothic, which means it’s probably from the 1350 to 1550 period.
Rose window, Sainte-Chapelle, c.1490, Paris
Rose window, Sens Cathedral, c.1516 — Photo by wiki user Pline
From the outside, the Flamboyant rose window of the Meaux cathedral looks like this:
Rose window, Meaux Cathedral, 14th century — Photo by wiki user Vassil
Sometimes, you can find both styles in the same cathedral, because it took such a long time to build that styles changed before it was finished. In that case, the Rayonnant part was probably built earlier than the Flamboyant part.
Of course, there are other Gothic styles and variations that developed from the 12th to the 16th century, but the Rayonnant and Flamboyant are the most spectacular, and they’re quite easy to spot.
The Rayonnant (1250-1350) is based on straight lines, like this:
By wiki user Benutzer
The Flamboyant (1350-1550) is even easier. It looks like flames: