Noun
square sail (plural square sails)
(nautical) A sail bent to a horizontal yard set athwartships.
As of 2015, it was still under thrust, proving the practicality of a solar sail for long-duration missions. citation It is spin-deployed, with tip-masses in the corners of its square sail. Source: Internet
No explanation is offered as to how this could be accomplished with a square sail as the lower reefed portion of the sail would be very bulky and would prevent even an approximation of the laminar flow necessary for windward sailing. Source: Internet