An interactive applet and associated web page that show the definition of a horizontal line in coordinate geometry. The applet has two points that the user can drag which define a line. The line flagged when it is horizontal (slope=0) and the equation of the line is shown.

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 horizontal, the line equation 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.