Device for determining an axis joining origins of at least two reference systems (Ref1, Ref2) of a rigid body of which the relative positions and orientation are not priorly known, in particular an axis passing through two points of a bone, the device comprising: at least a first and a second inertial sensor (1,2), provided with a triaxial accelerometer and a triaxial gyroscope, arranged to be constrained to said rigid body and for respectively defining the first and the second (Ref1, Ref2) reference system; a processing unit, connected to the inertial sensors (1,2) comprising: • an acquiring module configured to acquire first data representing angular velocities and second data representing linear accelerations of the rigid body detected by the first and second inertial sensor (1, 2) during a movement of the rigid body itself; • a first calculating module configured to calculate third data representing angular accelerations of the rigid body as a function of the first data acquired; • a second calculating module configured to calculate a relative orientation between the first and the second reference system (Ref1, Ref2) as a function of the first data; • a third calculating module configured to calculate a vector which joins origins (O 1, O 2) of the first and of the second reference system as a function of said first, second and third data (d1, d2, d3) and as a function of the relative orientation calculated between the first and the second reference system (Ref1, Ref2), wherein the primary direction (DIR_1) of the primary vector is representative of the axis joining the origins (O 1, O 2) of the two reference systems (Ref1, Ref2). at least a first and a second inertial sensor (1,2), provided with a triaxial accelerometer and a triaxial gyroscope, arranged to be constrained to said rigid body and for respectively defining the first and the second (Ref1, Ref2) reference system; a processing unit, connected to the inertial sensors (1,2) comprising: • an acquiring module configured to acquire first data representing angular velocities and second data representing linear accelerations of the rigid body detected by the first and second inertial sensor (1, 2) during a movement of the rigid body itself; • a first calculating module configured to calculate third data representing angular accelerations of the rigid body as a function of the first data acquired; • a second calculating module configured to calculate a relative orientation between the first and the second reference system (Ref1, Ref2) as a function of the first data; • a third calculating module configured to calculate a vector which joins origins (O 1, O 2) of the first and of the second reference system as a function of said first, second and third data (d1, d2, d3) and as a function of the relative orientation calculated between the first and the second reference system (Ref1, Ref2), wherein the primary direction (DIR_1) of the primary vector is representative of the axis joining the origins (O 1, O 2) of the two reference systems (Ref1, Ref2). • an acquiring module configured to acquire first data representing angular velocities and second data representing linear accelerations of the rigid body detected by the first and second inertial sensor (1, 2) during a movement of the rigid body itself; • a first calculating module configured to calculate third data representing angular accelerations of the rigid body as a function of the first data acquired; • a second calculating module configured to calculate a relative orientation between the first and the second reference system (Ref1, Ref2) as a function of the first data; • a third calculating module configured to calculate a vector which joins origins (O 1, O 2) of the first and of the second reference system as a function of said first, second and third data (d1, d2, d3) and as a function of the relative orientation calculated between the first and the second reference system (Ref1, Ref2), wherein the primary direction (DIR_1) of the primary vector is representative of the axis joining the origins (O 1, O 2) of the two reference systems (Ref1, Ref2).