The paper presents the design issues of support layer of timing belts. The construction of the power transmission belts needs different manufacturing process. This requires a proper design of the carrier layer, single or bifilar reinforcement cords and it affects the mechanical properties of the cord. In addition, there are various cord materials, different weave and diameters. Cord can be differently pre-impregnated with material of the belt or similar. Another aspects of the considerations are the inclusion of additional elements in the support layer, uneven distribution of the cord in the belt and intentional removal of cord or electrical cable entry.