There are only two eras. Simplex-era when had to check everytime if chain is aligned. First you do it by ear, but if it does not sound good, you do it visually

There are only two eras. Simplex-era when had to check everytime if chain is aligned. First you do it by ear, but if it does not sound good, you do it visually. [edit] This is btw why Eddy Merckx was constantly checking his crotch. And Shimano-era, when the selector has preset clicks. But wait. There was also brief moment of history, when the wire pulled towards smaller sprockets and strong spring pulled it uphill. But I dont remember the name of this system. [edit] The benefit of this system was that you can just flip the smallest gear on and the said spring will slowly guide the chain down.