In this paper we demonstrate that massless particles cannot be considered as the limiting case of massive particles. Instead, the usual symmetry structure based on semisimple groups like U (1), SU (2) and SU (3) has to be replaced by less usual solvable groups like the minimal nonabelian group sol 2 . Starting from the proper orthochronous Lorentz group Lor 1,3 we extend Wigner's little group by an additional generator, obtaining the maximal solvable or Borel subgroup Bor 1,3 which is equivalent to the Kronecker sum of two copies of sol 2 , telling something about the helicity of particle and antiparticle states.