Inherrently linked to the number three, the triangle possesses much of the same symbolism and meaning as does the number.

For Christians, it represents the Trinity (Father, Son, and Holy Ghost), while for the Celts it represented the Universe. Most systems recognize it as a symbol of family (Father, Mother, and Child) or balance.

For mystics, the triangle can also symbolize ley lines: Those channels of force found in most living things, including the Earth. The three sides are thus positive, negative, and neutral (the three sides together achieving the concept of balance). As long as these forces are in synch, they form a triangle which is not detectable. It is only where the forces are out of balance that manifestations of the energy appear. For ley lines, what you tap into depends upon where you are in the form (positive, negative, or neutral). Note also that "positive" here is not "good," nor is "negative" the same as "evil." These forces are more akin to magnetism.

Regarding superstition, this symbol is why you must never walk under a ladder or any such structure that creates a triangle: To walk through it may disrupt the lines of force and bring about bad luck. In the ladder scenerio, the ladder is positive, the wall negative, and the ground neutral -- all in perfect balance.