In many pictures of vintage pens, there are little bitty holes in the cap.

At first I thought this was where a missing clip used to attach. But now I've seen it in caps that have clips.

So, what are these little holes?

I believe those are breather holes. It prevents the cap from building up a vacuum or overpressure, causing the nib to leak or spurt in when opened.

I think that today's pens have them as well, they just then to be more hidden.

Ahhh, mystery solved. Thank you!
Now I'm going to search all over my modern pens to see if I can find the breather on them.

Hi SkinnyPens,

Yeah, the little holes (usually two) drilled into the cap are breather-holes. They're there to prevent air-pressure differences. Without the holes, uncapping a pen would cause a vacuum which would make the pen spit out ink. As I understand it, modern pens have these holes too, but they're a little more discrete. If you check under the clips, I think that's where they are. I've seen them on at least one of my modern pens.

You may not find a breather hole on a modern pen that threads onto the pen barrel. It can't be uncapped fast enough to create enough vacuum to pull ink out of the pen so it doesn't really need one. Some modern slip cap pens, like a Parker Sonnet, do have a breather. The Sonnet's breather is concealed under the ornament on the end of the cap. Most of my modern pens don't have breather holes.

I believe there was some US law in the 1930's and '40's that required breather holes in order to prevent - of all things - choking if the cap were swallowed. Clearly, the holes probably didn't work, but the companies had to cover their bets. Someone correct me if I'm wrong about this. I seem to remember reading something about it.

Best,
Summer Greer

That is probably why i have had problems with Parker 51's leaking into the cap.

I did not see any holes in the caps of my Cross Townsend pens.

Most of my other modern pens have screw caps, so they would not need the holes.