Usually after “short” vowels in words of more than one syllable.
ebb |
abbess abbot babbling blabbing blobbing blubber bobbin bobbing bubbler bubbling clobber clubber clubbing cobber cobbler cribbing dabbing demobbed dobber dobbin drabbest dribbling drubbing dubbing fibber fibbing fobbing gibbon gibbous gobbet gobbler gobbling grabbing grubber grubbing hobbit jabber jabbing lobbing lubber mobbing nibbler quibblers quibbling rabbi rabbit ribbing ribbon robber robbing rubber rubbing rubbish sabbath scabbard scribbler scrubber scrubbing slobber snobbish snubbing sobbing squabbled stabbing stubborn swabbing throbbing webbing wobbling yabby yobbo |