On compact and chiral strings

Closed bosonic strings in the target space Rd-1.1xT26-d and their Fock spaces are considered. There is discussed a compact string, which is a closed string moving in the Lorentz torus T25,1. Opposite to compactification procedure of decompactification and arising with the help of this procedure chiral strings are described. The problem of nonuniqueness of the string theory in the Rd-1, space and, in particular, in d=4 is discussed