The extended slit
One consistent characteristic of a good flexible nib is its long slit. Imagine yourself trying to bend the branch of a tree. It is nearly impossible from near the trunk. However, if you move out toward the end of the branch, bending becomes easier. In a sense, the same principle is at work with the long slit on the nib: the longer the slit, the greater the distance between the body of the nib and the tip, and so the more flexible the tines become. By extending the slit, and providing a second vent hole, the tines become longer, requiring less downward pressure on the paper to flex the tines.

The second vent hole
The slit is the weak point of any flexible nib. All the force that is applied to the nib is transferred to this area, testing the tensile strength of the body of the nib. To this end, the second vent hole is important in preventing the slit from continuing up into the body of the nib in the form of a crack or split.* The second vent hole, further away from the tip, has the effect of lengthening the tines as well.

One must allow this vent hole to interrupt the design of the imprint. On Pelikan pens it falls in the area below the Pelikan logo, showing that it is clearly an aftermarket modification.

This "Dos Ojos" nib, which has also received our added-flex customization, makes a great sketching instrument with lots of line width variation, as demonstrated by Danielle Behar, our fountain pen repair journeyman and artist..

It's good to note that the second vent hole is almost of no consequence in terms of the air return. The air return is controlled by the capacity of the air feed channel, not by the second vent hole.

When used on an appropriate 14K nib, in conjunction with other modifications, these nibs become quite flexible.