Load line

A load line is a single straight line drawn across a transistor's or diode's current-versus-voltage curves that captures everything the surrounding resistors and supply voltage will allow. Where that line crosses the device's own curve is the actual operating point — it is the graphical handshake between what the device 'wants' to do and what the rest of the circuit 'permits'.

Its two ends are easy: at one extreme the device is fully off (all the supply voltage appears across it, no current); at the other it is fully on (no voltage across it, maximum current set by the resistor). Sliding the bias along this line shows at a glance whether your Q-point sits in the useful middle or is jammed into saturation or cutoff — which is exactly why it is the classic tool for visualising biasing and signal swing.