When using a fountain pen, angles are important, and it is in this area that most people find themselves in trouble when they first start writing with one.  In considering angles, it’s worth thinking of the orientations of aircraft; they are the same general shape as pens, if one ignores the wings, and borrowing the jargon simplifies the task of describing correct grip

Roll: Imagine for a moment that the pen does have wings.  Writing, like flying, is much easier if the wings are level, and becomes nearly impossible as they come closer to vertical.

While turning a pen on its side doesn’t raise any dangers of plummeting to earth, it does prevent ink from getting onto the page.  With the pen in the correct degree of roll, the slit of the point is perpendicular to the paper, and the ink makes contact easily and flows out readily.  If the slit is rolled away from the paper, the ink can’t make contact, and without that connection there’s no writing.