The First Navy Jack is the current U.S. jack authorized by the United States Navy. The design is traditionally regarded as that of the first U.S. naval jack flown in the earliest years of the republic.
In late 1775, as the first ships of the Continental Navy readied in the Delaware River, Commodore Esek Hopkins issued, in a set of fleet signals, an instruction directing his vessels to fly a "striped" jack and ensign. The exact design of these flags is unknown. The ensign was likely to have been the Grand Union Flag, and the jack a simplified version of the ensign: a field of 13 horizontal red and white stripes. However, the jack has traditionally been depicted as consisting of thirteen red and white stripes charged with an uncoiled rattlesnake and the motto "Dont [sic] Tread on Me"; this tradition dates at least back to 1880, when this design appeared in a color plate in Admiral George Henry Preble's influential History of the Flag of the United States. Recent scholarship, however, has demonstrated that this inferred design never actually existed but "was a 19th-century mistake based on an erroneous 1776 engraving".
In 1778, John Adams and Benjamin Franklin wrote a letter to the Ambassador of the Kingdom of Sicily, thanking him for allowing entry of American ships into Sicilian ports. The letter describes the American flag according to the 1777 Flag Resolution, but also describes a flag of "South Carolina, a rattlesnake, in the middle of the thirteen stripes."
The rattlesnake had long been a symbol of resistance to the British in Colonial America. The phrase "Don't tread on me" may be coined during the American Revolutionary War, a variant perhaps of the snake severed in segments labelled with the names of the colonies and the legend "Join, or Die" which had appeared first in Benjamin Franklin's Pennsylvania Gazette in 1754, as a political cartoon reflecting on the Albany Congress.
The rattlesnake (specifically, the Timber Rattlesnake) is especially significant and symbolic to the American Revolution. The rattle has thirteen layers, signifying the original Thirteen Colonies. And, the snake does not strike until provoked, a quality echoed by the phrase "Don't tread on me." For more on the origin of the rattlesnake emblem, see the Gadsden flag.