the.com/crux

The one knot where everything tightens, the moment that decides the whole climb.

means The crux is the central, decisive point of a problem or matter — the single difficulty on which everything else hinges.

from Straight from Latin crux, meaning 'cross' — the same word that gave us 'crucify' and 'crucial.' The leap to 'decisive difficulty' likely owes something to crux interpretum, a scholar's phrase for a passage so tormenting it 'crosses' or torments the interpreters; the figurative English sense of a puzzling, pivotal sticking point took hold by the 19th century.

latin root
Means cross, the same word as crucifixion.
climbing term
Names the hardest move on any route.
southern cross
Crux is a real constellation guiding sailors south.
the pivot
In arguments, it's where the whole case stands or falls.
for instance

southern cross constellation — the smallest constellation, visible from southern hemisphere, contains the brightest stars alpha and beta crucis

crux biscuits — indian snack brand founded 1968, now owned by itc, most popular wheat biscuit in india

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