While contemporary Passacaglias evidence a logical formal organization, Bach's raises an analytical problem for which various solutions have been proposed. In order to clarify this complex situation, the paper first realizes a linear reading of the twenty one variations (theme included). It then proposes a plan in five groups, of five, four, three, four and five variations respectively. This plan is considered in detail, in order to show the symetries between the two groups of five and between the two groups of four variations, as well as the concentric disposition of the central group. Bach appears to have endeavored to attenuate the readability of the formal signals of this plan, to veil too marked cuts and to melt them in a continuous or graduated stream, in order to realize an equilibrium between the clarity of the formal plan and the continuous flux that characterize the genre of the passacaglia.