We have been taught at school that when a word ending in "LL" helps form a compound word, "LL" becomes "L" (e.g. skill -> skilful). I have also come across the usage of this adjective as skillful (with double L's). I checked the NGram to see how the usage has changed over time, and it got me more confused:
What is the correct form of skilful and similar words? Is there a definite rule or are both usages acceptable?