The lines a chiromant reads are flexion creases, and they have an anatomy of their own that is independent of anything read from them. The primary creases form in the womb, around the twelfth and thirteenth weeks, as the hand begins to fold; they mark where the skin is tethered to the fascia beneath so that it hinges cleanly instead of bunching. They are set before birth and change very little afterward, which is the first thing a reader should know about them.
What the tradition calls the Head Line is the proximal transverse palmar crease, the second of the three primaries. Where it and the distal crease fuse into one, the result is the single transverse palmar crease, a well documented variant found in roughly one per cent of the general population and much more often in trisomy 21. That association is a clinical observation about development, not a chiromantic reading, and the two should not be run together.
It is worth separating chiromancy from dermatoglyphics, which is often confused with it and is a different subject. Dermatoglyphics studies the ridge patterns of the skin, the loops and whorls used in fingerprinting, and it has a genuine research literature including work on developmental correlates. Palmistry reads the flexion creases, which are a different structure entirely, and there is no body of evidence connecting crease form to character, career or lifespan. The atlas records what the chiromantic tradition says about this line because that tradition is a real object of study. It does not endorse the reading.