  In this paper, we recall the linear version of the lattice Boltzmann schemes
in the framework proposed by d'Humi\'eres. According to the equivalent
equations we introduce a definition for a scheme to be isotropic at some order.
This definition is chosen such that the equivalent equations are preserved by
orthogonal transformations of the frame. The property of isotropy can be read
through a group operation and then implies a sequence of relations on
relaxation times and equilibrium states that characterizes a lattice Boltzmann
scheme. We propose a method to select the parameters of the scheme according to
the desired order of isotropy. Applying it to the D2Q9 scheme yields the
classical constraints for the first and second orders and some non classical
for the third and fourth orders.
