Use the Straightedge tools to construct segments, rays, and lines.
A straightedge is a ruler without marks. It can be used for drawing straight lines but not for measuring.
There are three Straightedge tools: the Segment tool, the Ray tool, and the Line tool.
When Sketchpad starts, the active Straightedge tool is the Segment tool. Choose a different Straightedge tool by pressing and holding the Straightedge tool icon in the Toolbox. When you press and hold, a menu pops out and you can choose any of the three tools.
If a Straightedge tool is already chosen, you can switch to a different Straightedge tool by holding the Shift key and pressing either the right or left arrow key.
Each straight object constructed by one of these tools is determined by two points. Use the tool in either of two ways:
•Click the location of the first point and then click the location of the second point, or
•Press at the location of the first point, drag to the location of the second point, and release.
The color and style of a new straight object is determined by Sketchpad's Preferences or by the last straight object for which you set the color or style.
Use the Segment tool to construct segments. 1.Choose the Segment tool in the Toolbox if it’s not already active. 2.Click to locate the first endpoint of the segment. (Click in empty space, on an existing point, on a path object such as a segment or circle, or on an intersection.) 3.Click again to locate the second endpoint. 
Use the Ray tool to construct rays. 1.Choose the Ray tool in the Toolbox if it’s not already active. 2.Click to locate the endpoint of the ray. (Click in empty space, on an existing point, on a path object such as a segment or circle, or on an intersection.) 3.Click again to locate the point through which the ray travels. (This point is called the through point.) To construct a ray that bisects the angle formed by three selected points or by a selected angle marker, choose Construct  Angle Bisector. 
Use the Line tool to construct lines. 1.Choose the Line tool in the Toolbox if it’s not already active. 2.Click to locate a first point through which the line travels. (Click in empty space, on an existing point, on a path object such as a segment or circle, or on an intersection.) 3.Click again to locate a second point through which the line travels. To construct a line that’s parallel or perpendicular to an existing straight object, use Construct  Parallel or Construct  Perpendicular. 
There are several commands in the Construct menu that construct straight objects without using the Straightedge tool. •Construct  Segment, Construct  Ray, and Construct  Line commands construct straight objects determined by two or more selected points. •Construct  Perpendicular Line and Construct  Parallel Line construct a line through a selected point that is perpendicular or parallel to a selected straight object. •Construct  Angle Bisector constructs a ray that bisects the angle formed by three selected points or by an angle marker. 
While you’re constructing a straight object, you can hold down the Shift key to make the object horizontal, vertical, or at an angle of 15°, 30°, 45°, 60°, or 75°. Construct the second point before you release the Shift key. 
To attach either the first or second determining point of a straight object to an existing object, use the Straightedge tool to click: •an existing point; •a path object such as a segment, a line, a circle, or an arc; or •an intersection of two path objects. When the Straightedge tool is in the right place to click an existing object, that object is highlighted (appearing thicker and in a special color) and identified in the status line. Be careful when you want to attach a defining point to an existing object. It’s not enough to position the tool so that the ray or line appears to go through the desired location. You must actually position the tool itself over the object to which you want to attach before clicking or releasing the button to locate the defining point. For example, you may want to draw a diagonal line passing through the intersection of a horizontal line and a vertical line. In the first illustration the diagonal line appears to pass through the intersection, but because the pointer is positioned to click in empty space, the second defining point of the line won’t be attached. In the second illustration the pointer is over the intersection and the intersecting lines are highlighted. Because the pointer is positioned correctly, the diagonal line will be attached to the intersection.
In another example, you may want to attach a ray’s through point to a circle so you can animate the through point around the circle. Positioning the pointer to click as shown in the first illustration won’t work because the through point will not be attached to the circle. Instead, click as shown in the second illustration (with the pointer over the circle and the circle highlighted) to attach the point to the circle.

See also:
How to Construct a Segment of Fixed Length