In most countries, they usually take the shape of an equilateral triangle with a white background and a thick red border.
However, both the color of the background and the color and thickness of the border varies from country to country.
The early signs did not have high-contrast lettering and their messages might have been easily overlooked.
Signs were written in the local language (example); symbolic signs, though long used on certain tradesmen's signs (like the pawnbrokers' tri-ball symbol) were to be used for traffic only much later in history.
Complex signage systems emerged with the appearance of motorcars.
Some other countries also use these standards for some signage. In Europe they are based on the UNECE Vienna Convention on Road Signs and Signals.
In the United States they are based on the MUTCD standard and often contain text only.
Some of the first roadside signs —ancient milestones— merely gave distance measures.
Hazard warnings were rare though occasional specimens appeared, such as the specific warning about horse-drawn vehicles backing up which was carved in stone in Lisbon's Alfama neighborhood in 1686.
The polar bear warning sign in Svalbard recently changed from displaying a black bear on white background to a white bear on black background (both signs are triangular with a red border).
Some countries (like France, Norway, Spain) that normally use a white background have adopted an orange or amber background for road work or construction signs.
Warning signs in some countries have a diamond shape in place of the standard triangular shape.
In the United States, Canada, Mexico, Thailand, Australia, Japan, Indonesia, Malaysia, New Zealand, most of South America, and also Ireland (diverging from the standards of the rest of Europe) use warning signs are black on a yellow background and usually diamond-shaped, while temporary signs (which are typically construction signs) are black on an orange background.