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Alternate interior angles are formed when a line, called transversal, cuts or intersects two or more lines.

In the figure below, line p is the transversal or the line that cuts or intersects two lines ( line m and line n ).

Alternate interior angles are on opposite sides of the transversal and on the inside of the lines the transversal cuts or intersects.

Angle a and angle b are on opposite sides of the transversal or line p and they are located inside line m and line n. Therefore, angle a and b are alternate interior angles.

By the same token, angle c and angle d are on opposite sides of the transversal or line p and they are located inside line m and line n. Therefore, angle c and d are also alternate interior angles.

When a transversal intersects two lines, two pairs of alternate interior angles will always be formed.