Isn't 'baker's dozen' an interesting expression. Dozen may be one of the earliest primitive groupings. It is also known as a devil's dozen or long dozen.
The expression came about with baked goods, as a way for bakers to avoid being blamed for shorting their customers. Bakers had to sell goods by the dozen at a specific weight. The expression evolved over the centuries to further variations around this intent.
While most things aren't sold by the dozen anymore, we still an buy a dozen eggs. This is common everywhere. They started out being sold by the dozen to match up with the Anglo-Saxon and Roman systems of measurement. One egg could be sold for a penny or 12 for a shilling. This persisted in the American colonies, after the revolution. And it just stayed after that.
What else is in the dozen group? 12 hours on a clock face 12 months in a year 12 signs of the zodiac 12 grades under college level 12 Tribes of Israel 12 Apostles of Jesus Sold by the dozen: roses, donuts, batteries 12 Days of Christmas 12 Olympians 12 Jurors on a jury
Our Valentine's pictures today started at the Ringling Museum in Sarasota.