Usually after “short” vowels.

VC and CVCCCVCWith consonant digraphsAfter vowel sound /ar/ spelt a (in Aust and southern UK accent)Polysyllabic words
ass (donkey)
bass (fish)
Bess
boss
Cass
cuss
doss
fuss
goss
Hass (avocado)
hiss
Jess
joss
kiss
lass
less
loss
mass
mess
miss
moss
muss
Ness
piss
poss
sass
suss
Tess
toss
Yass
 
But s as in bus
(last column)
bless
bliss
crass
cress
cross
dress
dross
floss
gloss
press
stress
chessbrass
bypass
class
glass
grass
pass
surpass
abbess
access
across
actress
address
amass
amiss
assent
assert
assess
asset
assist
assort
assume
basset
blossom
bossy
carcass (US)
caress
cissy
confess
cosset
darkness
dressy
duress
egress
emboss
endless
essay
excess
express
fitness
fortress
fossil
fussy
glossy
goodness
gossip
harass
harness
illness
impress
jackass
madness
mattress
morass
possess
possum
princess
process
profess
progress
recess
regress
repress
ruthless
sadness
sickness
success
suppress
unless
weakness
wireless
witness