How to tell how many generations of Apple phones are there?

The most obvious is the difference between shape and size. From the appearance, the iphone screen icons of the 5th generation are all five elements+one line. Four generations ago, the icons on the iphone screen were four lines+one line. The back cover of mobile phones before the 3rd generation was curved, and it was flat at the beginning of the 4th generation. Iphone5 and home button are the same as iphone4, and the home button of iphone5S has a circle.