  We present a thorough classification of the isotropic quantum walks on
lattices of dimension $d=1,2,3$ for cell dimension $s=2$. For $d=3$ there exist
two isotropic walks, namely the Weyl quantum walks presented in Ref. [G. M.
D'Ariano and P. Perinotti, Phys. Rev. A 90, 062106 (2014)], resulting in the
derivation of the Weyl equation from informational principles. The present
analysis, via a crucial use of isotropy, is significantly shorter and avoids a
superfluous technical assumption, making the result completely general.
