In the example, calculating the equivalent voltage: V T h = R 2 + R 3 ( R 2 + R 3 ) + R 4 ⋅ ⋅ V 1 = 1 k Ω Ω + 1 k Ω Ω ( 1 k Ω Ω + 1 k Ω Ω ) + 2 k Ω Ω ⋅ ⋅ 15 V = 1 2 ⋅ ⋅ 15 V = 7.5 V (Notice that R 1 is not taken into consideration, as above calculations are done in an open-circuit condition between A and B, therefore no current flows through this part, which means there is no current through R 1 and therefore no voltage drop along this part.)