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

ammo

brimming

bummer

clammy

comma

command

commence

commend

comment

commerce

commit

commode

common

commune

commute

cramming

dimmer

dimmest

dimming

drummer

drumming

dummy

flummox

gamma

gammon

gammy

Gemma

gimme

gimmick

glimmer

glummest

grammar

grimmest

grommets

hammer

hammock

hemming

hummer

humming

hummock

immense

immerse

immune

immured

Jimmy

lemming

mammal

mammon

mammoth

mummy

plummet

pommel

pummel

shimmer

simmer

skimming

slammer

slamming

slimmer

slimming

slumming

spammer

spamming

stammer

stemming

strumming

summer

summit

summon

swimmer

swimming

Tommy

tramming

trimmer

trimming

tummy

whammo

yammer