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Hi Herbert,
Look at https://www.flexpetz.com/dog-fence-planning-twisted-wire

"The most misunderstood part of the electric dog fence is twisted wire. At its core, twisted wire is actually pretty simple. Twisted wire is (as the name alludes to) 2 dog fence wires twisted together. You can purchase pre-twisted wire (we recommend this) or, with a little elbow grease and some spare time, twist your own wire.

Twisted wire plays an indispensable role in the function of your electric dog fence. Twisting two wires together creates a neutral wire that does not activate the warning or correction on your dog's collar. The twisted wire carries the signal from your transmitter box out to the main loop but remains neutral and therefore cross-able by your dog. Without the twisted wire your dog would not be able to cross the line that runs from your transmitter out to the boundary loop (see example images below). You can also use twisted wire to connect smaller loops to the main loop. Small loops can be created to keep your dog out of flower gardens, swimming pools, and just about any other no-go zone in your yard. "

This is probably under Dog Navigation.
Regards, Noell