There are 4 different P series digital pianos, 3 of them are full length 88 keys and 1 is 73 keys.The P45 is the first weighted digital piano that Yamaha do and comes in as the cheapest, weighted keys mean they are just like an acoustic piano! Which is great for beginners, and you get fantastic value!The P125 is the next model up, it has a much cleaner and quality sound for someone at an intermediate level. The piano has outputs so you can plug them into external speakers, which is great if you are performing at concerts!The P121 has only 73 keys, the main advantage of this is the portability, you can move around anywhere which is fantastic for gigging musicians!The P255 is top of the range in Yamaha's current portable digital piano range, a fantastic home and gigging piano. Improved speakers and improved key quality are new features on this one. The functions are also easily adjustable.