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Plane Geometry deals with points, lines, angles and polygons.

Line in geometry is an indefinitely long line without beginning or end.

The point can be the beginning of an indefinitely long line and this line is called a ray. A ray has a starting point, but no end.

If the line has two end-points, it is called a segment. See figure 1.

Figure 1.

When two or more lines intersect with each other, they form acute and obtuse angles. If the angle is less than 90 degrees, it is referred to as acute. If the angle is greater than 90 degrees, it is referred to as obtuse.

Two intersecting lines form four angles. Opposite angles are called vertical angles. Angles that share one side, are called adjacent angles. See Figure 2.

Figure 2.

Vertical angles 1 and 2 are equal. Adjacent angles 1 and 4 together form a straight line and add up to 180 degrees.

Angles 1 and 2 in Figure 2 are obtuse. Angles 3 and 4 are acute. If the angle is 90 degrees, it is called a right angle. See figure 3.

Figure 3. A right angle