Why do stamps have ridges on the sides?

Just wondering if they serve any sort of purpose - why not just make stamps square?Why do stamps have ridges on the sides?
So the sheets of stamps wouldn't curl.......





The rotary press printing method provided many advantages in the manufacture of U.S. postage stamps, primarily in the speed of production allowing the printing of stamps on rolls of paper rather than on the single sheet at a time of the flat plate method. It also allowed the gumming of stamps as a roll rather than by the individual sheet, again a big time-saver.





However, when the rotary press rolls were cut down to sheets to be delivered to post offices, the sheets tended to curl back into their rolled form. This was not a problem with rotary coils which were sold in rolls, but it was a problem for postal clerks who needed the sheets of stamps to lie flat in their drawers. More pressing was the fact that it was exceptionally difficult to add pre-cancels to the curled sheets without tearing the sheets apart at the perforations, a problem that was alleviated somewhat as the Bureau took over the pre-canceling chore.





Since un-gummed sheets from the rotary rolls did not curl, it was noted that the gum must be the cause of the curling. By simply adding ';breaks'; in the gum the curling was reduced considerably.Why do stamps have ridges on the sides?
I'm not 100% sure, but I have a guess. Stamps used to come in big sheets. The ridges worked as perferation...so that made them easier to seperate without needing scissors.
Stamps have perforations so that you can seperate them easily. The first stamps indeed came imperforate, and not until 1854 did they create a rotary press which could create the perforations. The perforations allowed vending machines to be created.





Now that most stamps are self-stick, the ridges remain - not as a needed feature but to maintain the classic stamp look.
When postage stamps are printed in large tear off sheets there is a line of perforations between each stamp.


When torn apart from the sheet the stamp has a line of ridges around it's edge.


It's the same as when you tear a page from a spiral notepad.
It is an homage to saltine crackers, that now come pre-broken, but used to come in sheets of four.





It is like the ridges on some coins to detect tampering and filing the edges down to collect some of the silver and gold. It also prevents wear on the edges of coins and stamps.





It is similar to the reason that we still 'dial' a telephone.
It makes stamps easier to remove from their backing