A correspondent writes to ask why we write full and not beautifull, etc.
There's been vacillation between -ll and -l for the suffix since Anglo-Saxon times. The suffix derives from full, but that word was often spelled with a single -l in Old English. As time went by, that -l became more frequent, perhaps because it was in an unstressed syllable and people were losing their sense of its original meaning of 'fullness'; but -ll remains the commonest spelling in the Middle Ages.
There is no clear pattern. Each word has its own history. Wonderful had -l in Old English and alternated with -ll until the 18th century. Beautiful had -ll until the early 1700s. Graceful was mainly -ll to begin with, sometimes -l, then settled down with -l from the mid-17th century.
The uncertainty isn't restricted to this particular suffix. Many other kinds of word were affected, such as natural and angel. And the uncertainty was still there even in Dr Johnson's time. In his Dictionary (1755) we find downhill and uphil, as well as downfall and pitfall and several others. The variation is still with us today, especially between American and British English: compare fullness / fulness, distill / distil, enroll / enrol, and so on.