Usually after “short” vowels in words with more than one syllable.

add

odd

Rudd

adder

addict

adding

address

adduce

bedding

bidden

bidder

bidding

bladder

budding

cheddar

cladding

cuddling

diddling

dodder

faddist

fiddler

fiddling

fodder

gladden

gladdest

goddess

haddock

hidden

huddling

kidding

ladder

madden

madder

maddest

meddling

midden

middling

muddling

nodding

oddball

odder

oddest

oddment

oddness

paddling

paddock

plodded

plodder

plodding

prodded

prodding

pudding

puddling

redden

reddest

reddish

riddance

ridding

sadden

sadder

saddest

shedding

shredded

shredder

shudder

skidding

sledding

sodden

studded

studding

sudden

thudded

thudding

toddler

trodden

udder

wadding

waddled

waddling

wedding