Box of four calls can be done from a tandem-based triangle. The apex effectively
backs up and plays the role of a trailer. The spot in front of him is a phantom.
The call completes in a 4-person setup with a phantom. If the result is a 1x4,
the phantom spot must be on the end. In this case, the setup is turned into
a 1x3 — the phantom spot goes away. If the result is a 2x2, there must be an
unambiguous person in tandem with the phantom spot. That person moves 1/2 position
forward or back to become the apex of a new triangle. If the result is a diamond,
the phantom spot must be a point. That spot goes away, and the result is the
triangle formed by the other 3 people.
\pwtriangle{b1n,b2s,g2s}
\phthree{b2n,b1s,g2n}
before Triangle Peel and Trail
after
\pwtriangle{b1n,b2s,g2s}
\pntriangle{b1e,g2w,b2w}
before Triangle Bingo
after
The preceding is the "formal" definition of the concept. That
definition is way more general than is acceptable in practice. In
fact, it is clearly defective — "Triangle Circulate" is ambiguous.
Exactly where to draw the line between acceptable and unacceptable
uses has never been worked out. Most callers stick to Peel Off types
of calls.