When the line slopes are $m_1$ and $m_2$, the acute angle satisfies an heta=left| rac{m_2-m_1}{1+m_1m_2} ight|.

This connects a visual intersection angle to the algebraic steepness of each line.

If $m_1m_2=-1$, the denominator becomes zero and the lines are perpendicular, so the angle is exactly $90^circ$.

If the slopes match, the lines are parallel and the intersection angle drops to $0^circ$.