Theory:

A quadrilateral is a four sided polygon formed by joining four line segments. It has four sides, four vertices and four angles.
output-onlinepngtools.png
 
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 \(\angle A\)