Electric susceptibility vs relative permitivity

[Nikitin]

So for dielectrics, electric susceptibility (χ) is just the relative permittivity of the dielectric (κ) minus one?
=> χ = κ -1?

Why? Doesn't this mean χ is a completely pointless constant? Just a different way to write κ?

 

[DrDu]

Yes, it is simply sometimes convenient not to have to write -1 all the time.

 

[Meir Achuz]

$${\bf P=\chi E}$$.
$${\bf D=\kappa\epsilon_0 E}$$.
$\chi$ and $\kappa$ are used in different relations.