The role of the two postulates of special relativity

Students are often mystified by the reasoning that leads from the postulates of special relativity to the requirement of covariance. This is partly due to the lack of transparency resulting from the failure to separate clearly the roles played by the postulates of the constancy of the speed of light and of relativity. Their roles are elucidated here by mainly geometric means. Finally some standard derivations found in textbooks are analyzed in order to sort out the basic physical ingredients.