[color=#0000ff][b]Straightedge[/b] [/color] can be regarded as a ruler without any marking.[br][br]The following are exactly what a straightedge can do:[br][list][br][*] Draw a unique straight line through two distinct points[br][*] Extend a line segment arbitrarily far in both direction[br][/list][br]Notice that it [b]cannot[/b] be used for measuring length.

[color=#0000ff][b]Compass[/b][/color] is a V-shaped drawing tool with a sharp point on one arm and a pencil on another arm. The two arms are joined by a hinge so that the openings of the arms is adjustible.[br][br]Obviously, a compass is mainly used for constructing a circle centered at any given point with any given radius.[br][br]There are two types of compasses:[br][list][br][*] [b]Modern compass[/b] - We can keep the opening fixed when the compass leaves the plane and carry to another location for construction.[br][*] [b]Euclidean (or collapsible) compass[/b] - The compass "forgets" the width of the opening when the compass leaves the plane i.e. we cannot keep the opening fixed.[br][/list]

We can use the following applet to simulate geometric constructions using straightedge and compass.