An interactive applet and associated web page that show the definition of a vertical line in coordinate geometry. The applet has two points that the user can drag which define a line. The line flagged when it is vertical (slope=undefined).

The grid, details and coordinates can be turned on and off. The applet can be printed exactly as it appears on the screen to make handouts.

The web page has a discussion on how to test for vertical and has links to other pages relating to coordinate geometry.

Applet can be enlarged to full screen size for use with a classroom projector. After use in the classroom, student can access it again from any web browser at home or in the library with no login required.

This resource is a component of the Math Open Reference Interactive Geometry textbook project at http://www.mathopenref.com.