This is one of my favourite letter-halakhot, from the rules of how to make straight nun:
|אות נו”ן פשוטה תואר צורתה כמו זיי”ן וג’ תגין על ראשה אך שהיא ארוכה כשיעור שתהא ראויה להעשות נו”ן כפופה אם תכפפנה||Its form is like a zayin, with three tagin on its head, but it is long, such that one could make bent nun out of it if it were bent round…|
And from the rules of straight khaf, clarifying the point1:
|שאין חילוק בין פשוטה לכפופה רק שזה פשוטה וזה כפופה…||…there is no difference between the straight and bent form save that one is straight and one is bent…|
Many people have difficulty visualising (and remembering) this. I hope that the animation displayed here will help.
The very best sorts of people do mitzvot as soon as the opportunity presents itself, correct? And we read Hebrew from right to left, so surely we should put tagin on the right-hand head of letters such as tzaddi, which have more than one head? Like the image at left, in fact.
We don’t, though. We put them on the left-hand head, like the image at right. Why’s that?
Because if you put them on the right-hand head, they’d fall off. (Keset haSofer, 5:2, letter tet.)
And this is why we make the right-hand heads curvy and upward-tilted.