What is used to define an inertial frame in special relativity? Do I need to take one of the postulates as defining an inertial frame? What is bothering me is that I used to use Newton\'s first law to define inertial frames (a freely moving object will have a constant velocity). This can not be the definition in SR because I could use the galilean transformations and Newton\'s first law would still hold (yet these are not inertial frames according to SR). So how do you define an inertial frame in SR?