There's a lot to be said about the Rule of Thirds. But the latter derives from an older principle known as the Golden Mean, or Divine Proportion, which is based on a 1.6 to 1 ratio. This is the old ratio used for proportions and it was modified to accommodate the 1.5 to 1 ratio of the 35mm negatives, degenerating into the famous Rule of Thirds.
Here's some food for thought and the way our actual rule of thirds got started:
The Golden Mean or Divine Proportion
