Right Angle

• The exact angle is 90$°$.
• Only right angles have a small square between the two lines.

Acute Angle

• An angle smaller than a right angle.
• An acute angle is an angle smaller than 90$°$ (< 90$°$).

Obtuse Angle

• An angle greater than a right angle.
• Angle of deflection exceeds 90$°$ but does not exceed 180$°$ (90$°$< obtuse angle < 180$°$).

Reflex Angle

• An angle greater than the obtuse angle.
• The reflex angle exceeds 180$°$ but does not exceed 360$°$ (180$°$< reflex angle < 360$°$).