Cosimo I de' Medici was the second duke of Florence from 1537 until 1569, when he became the first grand duke of Tuscany, a title he held until his death.
Portrait of Cosimo I de' Medici, c. 1545
Cosimo I de' Medici at about 19 years of age (by Pontormo, c. 1538)
Cosimo I's coronation as Grand Duke of Tuscany, which happened in 1570, Rome.
Official portrait of Cosimo I de' Medici as Grand Duke of Tuscany by Giovanni Battista Naldini
Duke of the Florentine Republic
The Duca della Repubblica Fiorentina, rendered in English as Duke of the Florentine Republic or Duke of the Republic of Florence, was a title created in 1532 by Pope Clement VII for the Medici family, which ruled the Republic of Florence. There were effectively only two dukes of the Republic of Florence, Alessandro de' Medici and Cosimo de' Medici, the second duke being elevated to Grand Duke of Tuscany, causing the Florentine title to become subordinate to the greater Tuscan title.
Image: Alessandro the moor
Image: Agnolo Bronzino Cosimo I de' Medici in armour Google Art Project