The definition and properties of the perpendicular bisector are explored using a geometry applet.

Review
The perpendicular bisector of a line segment AB is a line that is perpendicular to AB and passes through the midpoint of segment AB.(see figure below).

To construct a perpendicular bisector, we may proceed as follows:
1 - draw two circles centered at A and B and with equal radii.
2 - connect the intersections P and Q of the two circles to make a line. This line is perpendicular to AB and passes through the midpoint M of segment AB.(see figure below).