Let us consider two vectors $\vec{A}$ and $\vec{B}$ that lie in a plane as shown in figure $(a)$.

The lengths of the line segments representing these vectors are proportional to the magnitude of the vectors.

To find the sum $\vec{A}+\vec{B}$, we place vector $\vec{B}$ so that its tail is at the head of the vector $\vec{A}$, as in figure (b).

Then we join the tail of $\overrightarrow{\mathrm{A}}$ to the head of $\overrightarrow{\mathrm{B}}$.

This line $\overrightarrow{O Q}$ represent a vector $\vec{R}$, that is the sum of the vectors $\vec{A}$ and $\vec{B}$.

Since, in this procedure of vector addition, vectors are arranged head to tail, this graphical method is called the head-to-tail method.

The two vectors and their resultant form three sides of a triangle, so this method is also known as triangle method of vector addition.