linchpin

essential foundation keystone mainstay pillar

anchor base mainspring

key element main ingredient

basis center core cornerstone criterion crux essence gist heart idea kernel keystone mainspring marrow measure motive nub pith principle root source spring standard substance

basic idea central idea

end exterior exteriority outside periphery result surface

basic idea basis center core cornerstone criterion crux gist heart idea kernel mainspring marrow measure motive nub pith principle root source spring standard substance

backbone bulwark pillar

anchor brace buttress crutch maintainer prop sinew staff standby stay strength supporter sustainer upholder

good right arm right-hand person