We first introduce the notion of reference frame, itself related to the idea of observer : the reference frame is, in some sense, the "Euclidean space carried by the observer". Let us give a more mathematical definition:… the reference frame is... the set of all points in the Euclidean space with the rigid body motion of the observer. The frame, denoted R , is said to move with the observer.… The spatial positions of particles are labelled relative to a frame R by establishing a coordinate system R with origin O. The corresponding set of axes, sharing the rigid body motion of the frame R , can be considered to give a physical realization of R . In a frame R , coordinates are changed from R to R ' by carrying out, at each instant of time, the same coordinate transformation on the components of intrinsic objects (vectors and tensors) introduced to represent physical quantities in this frame.