If two lines meet at a right angle (90[math]^\circ[/math]), the line are said to be perpendicular. Look at the example below. Since AB is perpendicular to CD, we say that AB [math]\perp[/math] CD, the symbol [math]\perp[/math] means "is perpendicular to". When one of the 4 angles is a right angle, the other three angles will also be right angles.