  In 1909 Born studied the "relativistic undeformable body" but made the
mistake of calling it "rigid". The "rigid body" as one can find in Relativity
books is, in fact, this Born "undeformable body". In Relativity it is necessary
to distinguish between "rigid" and "undeformable". The "undeformable" body (in
the sense of the most rigid possible) must be the "deformable" body where
schock waves propagate with maximum speed c. We present in this text the
elastic laws for rigid bodies. We think that these laws, which are ignored by
the majority of relativists, should be taught in the elementary relativistic
courses. With the approach of 2005, the centenary year of Relativity, we should
like to appeal to all those who have some influence on these matters to avoid
this mistake of repeatedly calling "rigid" to the "undeformable body".
