Gules

In heraldry, gules is the tincture with the colour red. It is one of the class of five dark tinctures called "colours", the others being azure (blue), sable (black), vert (green) and purpure (purple).

Gules is portrayed in heraldic hatching by vertical lines, or indicated by the abbreviation g. or gu. when a coat of arms is tricked.

Etymology
The term gules derives from the Middle English goules, which itself is an Old French word meaning "neckpiece made of red fur". Goules is derived from the Old French gole or guele, both of which mean "throat", which are ultimately derived from the Latin gula, also meaning "throat". Gules is similar to the English word gullet. A. C. Fox-Davies states that the term originates from the Persian word gol, meaning "rose", but according to Brault there is no evidence to support this derivation.

The modern French spelling of the tincture is gueules. Both gules and rojo are used for red in Spanish heraldry. In Portugal, red is known as vermelho, and in Germany the colour is called rot. In Dutch heraldry, the tincture is called keel.

Poetic meanings
Centuries ago, arms were often described poetically and the tinctures were associated with different gemstones, flowers and heavenly bodies. Gules usually represented the following:
 * Of jewels, the ruby
 * Of heavenly bodies, Mars

Examples
Maurice of Nassau arms.svg coat of arms of Nassau-Dillenburg (attributed to Otto II of Nassau, d. 1351):

1. The lion of Nassau, Azure billetty or, a lion rampant of the last armed and langued gules ;

2. County of Katzenelnbogen, Or a lion rampant guardant gules, armed langued and crowned azure;

3. County of Vianden,  Gules, a fess argent;

4. County of Dietz,  Gules, two lions passants or armed and langued azure ]] Gules is the most widely used heraldic tincture. Through the sixteenth century, nearly half of all noble coats of arms in Poland had a field gules with one or more argent charges on them.

Examples of coats of arms consisting of purely a red shield (blazoned gules plain) include those of the d'Albret family, the Rossi family, the Swiss canton of Schwyz (prior to 1815), and the old coats of arms of the cities of Nîmes and Montpellier.