According to my 50-year-old textbook, two
phase is not used--it would need four wires.


Two-phase can be three, four or five wires, with four wires being the most
common.

Where the source is three-phase, a customer-owned Scott-T transformer is
usually used, in which case the secondary of said transformer is used to
"separately derive" the two-phase system, thereby allowing three wires to be
used for two-phase.

This would be the logical equivalent of grounded Delta three-phase, which has
only two ungrounded wires, yet delivers true three-phase.