What does adjacent mean for planes in a crystal lattice?

[ehrenfest]

Homework Statement

By book says prove that the distance between two adjacent parallel planes of a lattice is d(hkl) = 2pi/|G| where g = gb_1 +k b_2 + l b_3 where the b_i are the primitive vectors of the reciprocal lattice.

Could someone clarify exactly what they mean by "adjacent"?

Homework Equations

The Attempt at a Solution

 

[chemisttree]

"Next to" or 'alongside' or 'face-to-face'.