Two distinct planes in threespace either are parallel or intersecting in a line. If they intersect, you can determine the angle between them from the angle between their normal vectors.
Specifically, if vectors n_{1}=(A_{1},B_{1},C_{1}) and n_{2}=(A_{2},B_{2},C_{2}) are normal to two intersecting planes,
and
the angle θ between the normal vectors is equal to the angle between the two planes and is given by:
That is:

Consequently, two planes with normal vectors n_{1} and n_{2} are
 perpendicular if n_{1}·n_{2}=0
 parallel if n_{2}=kn_{1}, for some nonzero scalar k



Perpendicular planes 

Parallel planes 
Determine the angle between the planes
Solution:
n_{1}=(2,1,1) and n_{2}=(1,0,1)
θ=30º