The Triangle Law of Vector Addition, also known as the Triangle Rule or Parallelogram Law, is a graphical method used to add two vectors together to find their resultant vector. This law is particularly useful when dealing with vectors in two dimensions (in a plane). It's called the Triangle Law because it involves creating a geometric figure resembling a triangle or parallelogram.

Here's how the Triangle Law works:

1. Start with two vectors, let's call them vector A and vector B. These vectors have both magnitude (length) and direction.

2. Draw the vectors as arrows on a coordinate system or a piece of paper, placing them tail-to-tip. This means that the tail of vector B is placed at the tip of vector A, or vice versa.

3. Complete the parallelogram by drawing two more lines. One line starts at the tail of vector A and goes to the tip of vector B, and the other line starts at the tail of vector B and goes to the tip of vector A. This forms a closed figure, either a parallelogram or a triangle, depending on the original placement of vectors A and B.

4. The resultant vector, which represents the sum of vectors A and B, is the diagonal of the parallelogram or the third side of the triangle, starting from the common point where vectors A and B intersect.