To subtend an angle
is to form an angle at a point.
When you look at an object such as
a tree, the angle subtended at your eye is that formed by imagining
two lines drawn from the top and bottom of the object to your eye.
![](../images/subtend.jpg)
When two lines AP and BP are drawn joining the three points A, B and
P on a circle, then the angle APB is said to be subtended by the chord
AB or by the arc AB. All the angles formed on one side of a chord
of a circle in this way are equal to each other. The angles subtended
by a diameter at its circumference are all 90o.