"Gregory's Rule"
(Not to be confused with Pope Gregory's Pastoral Rule or Regula Pastorialis, a guide for monastery life)

       Medieval scribes routinely prepared their manuscripts so they would be as pleasing as possible to the senses of sight and touch.  At some point early in the history of the codex book, which replacd scrolls in the first eenturies of the Current Era, they discovered their completed codices would look better if they always copied their texts so that the collated leaves (folded and stacked in "quires" before binding) would be arranged so that any page opening (a verso of a leaf on the left and a recto on the right) would be arranged so that rougher hair-side leaves faced each other, and smoother lighter flesh-side leaves faced each other.  This prevented the juxtaposition of "hair-flesh" from drawing attention to the difference between the "substrate" or "page support," allowing the scribe's cherished scripts and colorful illuminations to command the reader's full attention.
       Though this practice seems to have been nearly universal in Europe and the British Isles during the medieval period, by the modern era, whcn fewer and fewer scholars were hands-on familiar with parchment MSS, awareness of the practice was lost.  The first modern scholar to draw attention to the hair-hair and flesh-flesh pairing seems to have been Caspar René Gegory (1846-1917).  He was an American scholar working in Germany where he studied textual criticism of the New Testament, that is, how the gospels etc. came to their current form by comparing manuscripts chronologically in order of creation.  His direct "autoptic" examination of many parchment manuscripts, rather than their surrogates in printed editions, enabled him to observe the practice and bring it to our attention.  After Gregory's work ended when he was killed in World War I while serving in the German army, having become a Saxon citizen during his studies, this scribal practice has been known as "Gregory's Rule" or sometimes "Gregory's Law."  As with so much to do with scribal practices, like the dramatic shifts in scripts from Carolignian to Gothic to Bastarda or the change (around 1210 CE) from starting pages below the top ruled line to starting them above it, we have no evidence of it ever being taught as a formal "rule" or "law."  They just handled parchment that way, probably having been taught during their apprenticeship along with everything else to do with their art.  Carpenters have similar traditions for handling wooden boards to minimize the curving effects of trees' natural growth rings upon the finished shape of a wall or floor (i.e., usually, you set all the boards with their curved grain facing up).  I did not read this in a carpentry book, but was taught it on the job while we were installing a pier's deck so that it would naturally shed rain rather than cupping it in puddles on each board.

Leila Avrin, Scribes, Script, and Books: The Book Arts from Antiquity to the Renaissance. Chicago; London: American Library Association; The British Library, 1991: 213.