Voltage divider
Two resistors in series turning a voltage into a smaller, proportional one: Vout = Vin · R2/(R1+R2). The simplest circuit that does real work — references, feedback networks, level scaling, sensor reading.
In practice
Two cautions: it only holds when lightly loaded (the load becomes part of R2), and it burns current forever — a divider across a battery is a standby drain. For feedback networks, total resistance trades noise immunity against wasted current.