Square-rigged boats could sail no higher than a beam reach. However, the sails
were not flat, and they were trimmed appropriately to allow the boats to be
sailed other than dead downwind.

I don't recall the date triangular sails were invented, but they were the dawn
of upwind sailing in the west; junk rigs enabled upwind sailing in the east.

"Axel Boldt" wrote...
Thanks a lot for the illuminating answers. I read somewhere that the
"sail-as-airfoil" trick is a rather recent one, and that formerly
people would just let the wind push them around. Is that true, and if
yes, how recent is the invention?