 There is arguably a tight relationship between the geometrical symmetries of a spacetime and the symmetries of a theory that describes the physics of matter (in a broad sense, including fields) in it. (Theories such as GTR, in which space-time has its own dynamics are more complicated, and will be discussed later; for further discussion of symmetries in physics, see the entry on  symmetry and symmetry breaking.)  Each symmetry is a set of transformations, with a rule of composition: formally a ‘group’. For instance, the group of rotations in the plane has a distinct element for every angle in the range 0–360 degrees; the composition of two rotations is the single rotation through the sum of their angles. Spacetime symmetries are those transformations which leave invariant a piece of spacetime structure (e.g., the symmetries of Minkowski spacetime are translations, spatial rotations and Lorentz boosts: together, the so-called Poincaré transformations); dynamical symmetries are those transformations which leave invariant a set of dynamical equations (e.g., the symmetries of Maxwell’s equations of electromagnetism are again the Poincaré transformations). There are good reasons to hold that the symmetry groups of theory and spacetime must agree. First, since the theory describes matter, and hence (arguably) what is measurable, any theoretical symmetries not reflected in the postulated spacetime structure indicate unmeasurable geometry: for instance, if an absolute present were postulated in relativistic physics. While in the other direction, if there were extra spacetime symmetries beyond those found in the dynamics, then per impossible one could measure nonexistent geometric quantities: for instance, a theory that depends on absolute velocities cannot be formulated in Galilean spacetime (see the entry on  absolute and relational space and motion: classical theories  for further discussion of these Newtonian spacetime structures). Famously, Earman (1989, ch. 3) declares that the matching of space-time and dynamical symmetries is, thus, an ‘adequacy condition’ on a physical theory.