What's With The Parallel Creases In Older Stamps?

Hi all, this is a fairly new hobby for me and I have a question regarding some old mint stamps (pre-1950ish) that I come across. Many seem to have parallel creases running through them, they are most obvious along the gum on the back, but visible from the front as well depending on the design/color. The most common stamps I see have two parallel creases separated by about 1cm. Others have three parallel creases, perpendicular ones, or some other pattern.

As an example, please see this ebay listing:
http://www.ebay.com/itm/252432086362

They are most obvious in the third photo (stamp gums).

What causes these creases and how common are they? I assume there is no trick to removing them? I'm putting together a collection of mint stamps - I hate the way these creases look but so many stamps seem to have them. When I order off ebay and bidstart, sometimes I get stamps with these same creases as well. I've ordered the same stamp 3-4 times and sometimes they all have the creases. They aren't always obvious from scanner photos. I'm wondering if it's time to give up and include this less-than-perfect stamps in my collection...

Thanks!

Those are not creases, they are called "gum breaker bars" and they show up on the gum of rotary press stamps.

Fascinating! I found a nice description here:
http://www.shaulisstamps.com/tips/GumBreakers.htm

Do these decrease the value of a mint stamp? Sounds like they are normal and expected, and that I shouldn't worry about them?

Thanks for your response!

They were put there intentionally, thus are normal and expected.

The absence or presence of gum breakers can occasionally serve as an identification tool. One example that comes to mind is Scott 1622 vs 1622C. The design for these two 1975 stamps is the 13-star flag over Indepence Hall. While the perfs are different, for a mint stamp you only need look at the back. 1622 has gum breakers whereas 1622C does not (the gum is flat).

It's more then occasionally, they are very useful throughout the late teens and early to mid 20's in making identifications easier. And they help greatly in determining genuine examples from fakes in the Kansas-Nebraska series.

Great, thank you for the responses!