They are referred to as "diamond-shaped". Geometrically, however, most US traffic warning signs are actually squares that have been rotated 45 degrees from horizontal, rather than diamonds, which have no right angles.
Many warning signs are also rectangular, such as a yellow sign with a black arrow indicating which way the road goes, or a black chevron indicating a curve.
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Warning signs for traffic are usually diamond-shaped (i.e., rhombus), or a square oriented with one corner pointed down.
Warning signs for other purposes may be square, rectangular, round or other shapes as may be required by local standards.
A yield or give way sign is typically in the shape of a triangle. However, they may be circular. You do need to check for your particular region as mentioned below. See link.
A warning sign is in the shape of a triangle and a caution sign is in the shape of a triangle too but it all depends on what type of caution sign really....hope i helped