One of my students is studying woodpeckers and when reading about the red-bellied woodpecker, it was described as "ladder-backed." He asked: Why are there two <d> s in the word ladder? Well, I had never thought of this question before. My student hypothesized that it could be the suffixing convention of doubling a final consonant when adding a vowel suffix, and that <-er> is a common suffix in English words. So, that passed the structure test.

It didn't make sense with the meaning though. A lad is a boy and that doesn't have anything to do with the meaning of the word ladder. And we couldn't find any other meanings for this word in the Mac dictionary.

So, we consulted etymonline:
ladder (n.)
Old English hlæder "ladder, steps," from Proto-Germanic *khlaidri (cf. Old Frisian hledere, Middle Dutch ledere, Old High German leitara, German Leiter), from PIE root *klei- "to lean" (cf. Greek klimax "ladder;" see lean (v.)). In late Old English, rungs were læddrestæfæ and the side pieces were ledder steles. The belief that walking under one brings bad luck is attested from 1787, but its origin likely is more pragmatic than symbolic. Ladder-back (adj.) as a type of chair is from 1898.

When we look at the entry in etymonline, we see that it is an Old English word. (Interesting that the Middle Dutch looks so similar to a Latin infinitive! I don't know anything about Dutch; I find no evidence of a Latin infinitive ledere.)

We were beckoned by the link to lean from the Greek, and found this:

lean (v.)
c.1200, from Old English hleonian "to bend, recline, lie down, rest," from Proto-Germanic *khlinen (cf. Old Saxon hlinon, Old Frisian lena, Middle Dutch lenen, Dutch leunen, Old High German hlinen, German lehnen "to lean"), from PIE root *klei- "to lean, to incline" (cf. Sanskrit srayati "leans," sritah "leaning;" Old Persian cay "to lean;" Lithuanian slyti "to slope," slieti "to lean;" Latin clinare "to lean, bend," clivus "declivity," inclinare "cause to bend," declinare "bend down, turn aside;" Greek klinein "to cause to slope, slant, incline;" Old Irish cloin "crooked, wrong;" Middle Irish cle, Welsh cledd "left," literally "slanting;" Welsh go-gledd "north," literally "left" -- for similar sense evolution, see Yemen, Benjamin, southpaw).

The word looks to be a free base with no affixes, so our question stands, why are there 2<d> s? I don't know enough about Old English or Greek to follow this through. The ol' Orten Gillingham would advise that this is to signal that the does not represent /eI/. Can anyone shed some light on this? Did the double <d> evolve to prevent the word from appearing to be complex?

Thanks for any input. My student is excited to hear back about his question.