Convert from mark twain to iron

Unit Definition (mark twain)
The mark twain or twain is an old word for the number two, derived from the Anglo-Saxon twegen. The American author Samuel Clemens (1835-1910), who had been a riverboat pilot on the Mississippi in his youth, took his literary name from a traditional riverboat phrase "mark twain", meaning "exactly two" fathoms of water. This was the minimum depth needed for the boats to operate safely without running aground.

Unit Definition (iron)
The iron is a traditional unit measuring the thickness of leather used in making shoes, especially the soles of the shoes. One iron is equal to 1/48 inch (0.5292 millimeter), so a sole 1/4 inch thick is described as "12 iron".