When that happens, we sometimes get more than just one new word; we get a new kind of word ending, one that goes on to a long, productive career in word formation. Bookmobile was born in the 1920s ...

When you add a suffixes and the second syllable of the new word is stressed, then you need to double the 'r'. For example, adding the suffix -ed to the word 'prefer': prefer + -ed → preferred ...