A concave hexagon is one in which one or two interior angles are greater than 180 degrees. With one reflex angle, it would look like a chevron.
The browser used here is pretty grim for graphics but, the following is a crude rendition. Ignore the dots - they are required to get the spacing approximately right!
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A concave hexagon or a concave octagon.
A concave hexagon is a hexagon that has one or two reflex angles. With one reflex angle, it will resemble a chevron - like a stripe on a police or army uniform, or the letter V which has thickness. With two reflex angles, it will resemble an hourglass with straight sides.
A concave irregular hexagon can have up to 6 right angles. A convex irregular hexagon can have up to 3 right angles.
It looks like four equilateral triangles that have been put together so that one vertex of each meet at a point.
None. Though there can be ten if the hexagons are concave hexagons.