Roy G. Biv is a traditional mnemonic for the sequence of hues in the visible spectrum, in simple rainbows, and in order from longest to shortest wavelength:
The colors are arranged in the order of decreasing wavelengths, with red being 650 nm and violet being about 400 nm.
Because the spectrum is a continuum, the selecting or omitting of individual colors in a list of representative colors is arbitrary. The traditional inclusion of the color indigo is attributed to Isaac Newton, who wanted the number of colors in his spectrum to come out to seven to match the number of days in the week, the number of notes in the major scale, and the number of known planets. He originally (1672) named only five primary colors: red, yellow, green, blue and violet; only later did he introduce orange and indigo . The Munsell color system, the first formal color notation system (1905), names only five "principal hues": red, yellow, green, blue and purple (though note that Munsell's purple is not a spectral hue).
The meaningless but speakable word ROYGBIV is also used as a mnemonic for the spectral colors, as is its slightly less euphonious reverse VIBGYOR.
Another traditional mnemonic device has been to turn the seven initial letters of the spectral colors into a sentence. In England probably the best known of these is "Richard Of York Gave Battle In Vain". The mnemonic is said to allude to the defeat and death of Richard, Duke of York at the Battle of Wakefield, a battle which Richard had himself started and which led to the defeat of his forces and his death as well as that of his son.
"Roy Great Britain the Fourth" is sometimes used, referencing again the name "Roy" along with the Great Britain national code (GB) and the Roman numerals for four (IV).
