Figure 1.6. There is an ideal voltage source on the left. The symbol is a circle with a plus sign near the upper, positive terminal. The bottom end of the source goes to the bottom line of the circuit. The voltage source is labeled V Th. The top end of V Th connects to the left end of a resistor labeled R Th. The right end of R Th connects to the upper output terminal. The lower output terminal connects to the bottom line of the circuit. End verbal description.