The Dutch tricolor is one of the oldest in Europe, and the odd thing about it is that it used to be orange.
From the Prince's Flag to red, white, and blue
The earliest known version dates to the Eighty Years' War (1568-1648), the long revolt against Spanish rule. It was the "Prince's Flag", associated with William of Orange, and it ran orange, white, and blue after the colors of William's coat of arms.
Orange gave way to red in the late 17th century. The reason is not settled. Some historians argue red was easier to make out at sea; others read the change as a shift in political allegiance. Either way, by the 19th century the red, white, and blue version was in general use, and on February 19, 1937, Queen Wilhelmina confirmed those colors by royal decree.
The flag itself
Three horizontal stripes of equal width: red at the top, white in the middle, blue at the bottom. The shades are specified as bright vermilion, white, and cobalt blue, and the ratio is 2:3. Naval and government standards fix exact color codes and dimensions so that official uses match.
Flying it
Government buildings and military institutions fly the flag, and the rules are taken seriously, particularly on King's Day, Liberation Day, and Remembrance Day. On royal birthdays it goes up at full mast with an orange pennant, a nod to the royal house. Private citizens fly it on national occasions under the same etiquette: it should not touch the ground, and it is not meant to be used as decoration or as clothing. Misuse draws public disapproval and occasionally legal trouble.
Relatives abroad
The tricolor pattern turns up in the flags of Luxembourg and Slovakia among others. Luxembourg's is the closest match, distinguished mainly by a lighter blue. Dutch activity overseas during the colonial period also left visual traces in parts of South America and the Caribbean.
The clearest descendant is Russia's. Peter the Great was building his navy on Dutch lines in the 1690s, and the merchant flag he settled on was the Dutch tricolor with the colors reordered. Through Russia the arrangement spread into the flags of the Slavic states. The French tricolor is often traced back to the Dutch example too, though its colors came from Paris and the Bourbon monarchy rather than from the Netherlands, and it runs vertically rather than horizontally. The Union Flag of the United Kingdom is not related at all: it predates the Dutch tricolor and is built from the crosses of Saint George and Saint Andrew.
References
[1] Rijksvoorlichtingsdienst (Dutch Government Information Service) - www.rijksoverheid.nl[2] Netherlands Institute for Military History - www.nimh.nl[3] Vexillological Association of the Netherlands - www.vexilla-mundi.org[4] Official royal decrees and historical records - koninklijkhuis.nl