The Penrose triangle, also known as the tribar is an impossible object devised by the mathematician Roger Penrose in the 1950s. It features prominently in the works of artist M. C. Escher, whose earlier depictions of impossible objects partly inspired it.

The tribar can only exist as a 2-dimensional drawing, since it exploits the ovelapping of parallel lines drawn in different perpectives. It appears as a solid object, made of three beams of square cross-section. All the beams are mutually perpendicular, yet join to form a triangle.

The concept can be extended to other polygons, making, for example the "Penrose square", but the visual effect is not as striking.

It is possible to create a solid object which looks like a Penrose Triangle. Such shapes can be either curved or have a break in them, but when viewed from a certain angle give the illusion of the complete triangle.

Link with pictures: http://www.cs.technion.ac.il/~gershon/EscherForReal/