Thevenin's theorem and Norton's theorem

Thevenin's theorem and Norton's theorem are collectively called the generator theorem, the active two-terminal network theorem, and the equivalent source theorem.

In engineering practice, we often encounter situations where we only need to study a certain branch. In this case, we can transform the rest of the circuit (usually a two-terminal network or a one-port network) except the branch we need to keep into a simpler source-containing branch (a voltage source and a resistor in series or a current source and a resistor in parallel), which can greatly facilitate our analysis and calculation. Thevenin's theorem and Norton's theorem provide the equivalent source-containing branch and its calculation method.