As stated above and ATP is an ATP. There are, however, two different written tests you can take but they get you to the same place. Usually a written test expires after two years if you have not completed the practical. My understanding is that this is not the case with the ATP. If you take the 121 written and are employed as a 121 pilot then the written with not expire. Same goes for the 135 written and you are employed as a 135 pilot. I believe. It is also my understanding that if you take the 121 written (like I did) and are employed as a 135 pilot (like I am) then the written WILL expire in two years. Usually I have the regs to defend my position but this time I don't. I'm hoping someone else can dig me out.