The sculptors of the twelfth century had made a calendar in stone on the western front; those of the thirteenth did the same in the right-hand doorway of the north porch, justifying this reduplication of the subject on the same church by the fact that the zodiac and the seasons may in symbolism have several interpretations.
According to Tertullian the death and new birth of the circling years afforded an image of the Resurrection at the end of the world.

According to others the Sun, surrounded by the twelve Signs, was emblematic of the Sun of Justice surrounded by his twelve Apostles.

The Abbe Bulteau sees in these stony calendars a rendering of the passage in which St.Paul declares to the Hebrews that "Jesus is the same yesterday, to-day, and for ever," while the Abbe Clerval gives this simple interpretation: that all times belong to Christ, and are bound to glorify Him.
"But this is a mere detail," said Durtal to himself.

"In the whole structure of the cathedral itself we can trace two-fold purposes.
"The architectural mass of Notre Dame de Chartres as a whole may be divided, externally, into three great parts, as indicated by the three grand porches.

The western or royal portal, which is the ceremonial entrance to the sanctuary, between the two towers; the north porch on the side next the bishop's palace, beyond the new spire; the south porch, flanked by the old spire.
"Now, the subjects represented on the royal front and in the south porch are identical.