By the way, I used two different gauges of wire because it seemed to me to be a good idea to have the same resistance for the two pairs of speakers.
The receiver is upstairs, and one pair of speakers is downstairs. To connect those speakers, I needed 80-ft wires. To minimize the resistance, I used 10-gauge wire.
The other pair of speakers is upstairs, close to the receiver. I found a formula somewhere for computing resistance. It turned out that 20 feet of 16-gauge wire produce the same resistance as 80 feet of 10-gauge wire.
Anyway, I don't know whether it was all that great an idea in the first place, but that's why I used two different gauges of wire.