The cathedral of Chartres has the largest glass complex of the 13th century. The collection has been preserved over time, and there is no other European equivalent: more than 80% of the original glass remains.
See: What remains of the XIIIth century glass? (pdf – French)
Events which could have caused irreparable damage and loss elsewhere had limited effects in Chartres: a single chapel was opened in the aisles at the end of the Middle Ages. The cathedral was not vandalized during the wars of religion.
The ‘modernizations’ of the 18th century favored clear, or non-stained glass. This reached eight high canopies in the choir while the revolution removed some low canopies from the transept.
During the First and Second World Wars, all the stained glass was removed to be placed in a safe place, thereby avoiding the blast effect of the explosives dropped on the city.
Recent analysis of the glass has shown that the authentic amount of XIIIth century stained glass is remarkably high.
See: When did some stained glass disappear? (pdf – French)