升降索通常从帆的前侧延伸到桅杆顶端，然后下降到甲板平面，在甲板上拉起升降索就能升起船帆。

The halyard typically rises from the head of the sail to a block at the head of the mast and back down to deck level, from where it is pulled up to raise the sail.

youdao

点Q0和Q1，在它的同侧还是异侧？，或者其中一个在平面上，抑或是不能确定？

This point, Q0 and Q1, are they on the same side, on opposite sides, is one of them on the plane, or we can't decide?

youdao

请说，所以我说当固定θ时,得到的是半平面，而非全平面,它布满z-轴的一侧,但从不延伸到z-轴的另一侧，为什么？

Yes? Yeah, so I'm saying when you fix theta you get only a half plane, not a full plane I mean, it goes all the way up and down, but it doesn't go back to the other side of the z axis. Why?

youdao