What is the X in X-mas?
doublet dou·blet (dŭb'lĭt)
A pairing of two lenses to optically correct a chromatic and spherical aberration.
chief upper garment worn by men from the 15th to the 17th century. It was a close-fitting, waisted, padded jacket worn over a shirt. Its ancestor, the gipon, was a tunic worn under armour, and at first it came down almost to the knees. The civilian doublet at first had skirts but gradually lost them. It had no collar until 1540, allowing the shirt to be seen at the neck; the shirt was also visible through slashes or pinking in the material.