 During the peak of the age of sail, beginning in the early 17th century, it was customary (and later a legal requirement) for ships to carry flags designating their nationality; these flags eventually evolved into the national flags and maritime flags of today.Flags also became the preferred means of communications at sea, resulting in various systems of flag signals; "see, International maritime signal flags".Use of flags outside of military or naval context begins only with the rise of nationalist sentiment by the end of the 18th century, although some flags date back earlier.