This root first appeared in The Etymologies of the 1930s as unglossed ᴹ√PEN with extended form ᴹ√PÉNED from which the main words in its entry were derived: ᴹQ. penda “sloping down, inclined”, ᴹQ. pende “slope, downslope, declivity” and N. penn “declivity” (Ety/PEN). The last of these has a precursor in Early Noldorin Word-lists from the 1920s: ᴱN. benn “inclined, sloping” or “slanting, sloping, up or down hill”, along with a noun form ᴱN. binn “slope”, apparently derived from the adjective plural (PE13/138, 160). This earlier form is reflected on several rejected roots in The Etymologies, with ᴹ√BEND >> ᴹ√DEN >> ᴹ√PEN (EtyAC/DAT, DEN).
In later writings, Tolkien generally gave the base root as √PED, a change Tolkien seems to have introduced to avoid conflict with a new root √PEN “lack, not have” (PE17/171, 173). The root √PED was variously glossed “incline, slope” (PE17/171), “fall in steep slant, incline, slope” (PE17/173) or “slope, slant down” (WJ/375); all these notes date to 1959-60. Tolkien went to say that “strong forms [were] lost in Quenya owing to similarity to √PER half” (PE17/173). Indeed, in this period like in the 1930s, all the actual derivatives (in both Quenya and Sindarin) seem to be based on √PEND-, so it may be easier to assume that this was the true form of the root.