### Theory:

A quadrilateral is a four sided polygon formed by joining four line segments. It has four sides, four vertices and four angles.

Vertices: $$A$$, $$B$$, $$C$$, $$D$$

Sides: $$\overline {AB}$$, $$\overline {BC}$$, $$\overline {CD}$$, $$\overline {DA}$$

Angles: $$\angle A$$, $$\angle B$$, $$\angle C$$, $$\angle D$$

Opposite sides: $$\overline {AB}$$ and $$\overline {DC}$$; $$\overline {BC}$$ and $$\overline {AD}$$

Adjacent sides: $$\overline {AB}$$ and $$\overline {BC}$$; $$\overline {BC}$$ and $$\overline {CD}$$; $$\overline {CD}$$ and $$\overline {DA}$$; $$\overline {DA}$$ and $$\overline {AB}$$

Opposite angles: $$\angle A$$ and $$\angle C$$; $$\angle B$$ and $$\angle D$$

Adjacent angles: $$\angle A$$ and $$\angle B$$; $$\angle B$$ and $$\angle C$$; $$\angle C$$ and $$\angle D$$; $$\angle D$$ and $$\angle A$$