If two lines are perpendicular, then they form right angles. If two lines meet at 90 angles. then they are perpendicular. Two lines meet at a 90 angle.

a
Conclusion: The lines are perpendicular and meet at a 90 angle.

b
Conclusion: The lines are parallel.

c
Conclusion: The lines form a right angle.

d
Conclusion: The lines meet at a 90 angle.