**If m1 = m2 **

**Both lines are parallel to each other.**

**θ = 0**

** 2. If m1m2 = -1 **

**Both lines are parallel to each other.**

**θ = 90**

**Two given line –**

**L1 :: a1x + b1y + c2 = 0 and **

**L2 :: a2x + b2y + c2 = 0 **

**Parallel Test :**

** If a1 : a2 = b1 : b2 ( both slope are equal m1 = m2 )**

**Perpendicular Test :**

**If a1a2 + b1b2 = 0 ( m1m2 = -1 )**

**Identical Test :**

**a1 : a2 = b1:b2 = c1:c2**