An angle in geometry is denoted by the angle symbol () followed by three letters which represent points that form the angle. For example the figure below illustrates the angle formed between two lines.
When referencing this angle in an expression or statement a mathematician writes . In plain language this means the angle formed by the three points , and with the vertex at point .
Congruent angles are denoted with tick marks across the angle.
Complementary angles can visually be denoted as two angles who sum to a perpendicular or square angle.
Supplementary angles can visually be denoted as two angles who sum to 180 degrees or PI degrees.