Davisson-Germer experiment and Bragg's law

[bubblewrap]

In Davisson-Germer experiment the necessity of constructive interference is dsin(theta)=n(wavelength) but in Braggs law its 2dsin(theta)=n(wavelength), why are these two different? I saw that Braggs law is used between layers of crystals but I've also seen Davisson-Germer experiment's equation being used in distances between crystals, including the distance between the layers.

 

[soarce]

Maybe the interatomic distance d is considered in a different way, from which book(s) did you get the diffraction laws ?