Boeing and Airbus call these a bit different but the principle stays the same.

Airbus has the following laws:

Normal law
Flight control Normal Law provides three axis control, flight envelope protection and manoeuver load alleviation. Normal Law operates in different modes depending on the stage of flight. These modes include: Ground Mode, Flight Mode, Flare Mode

Alternate law
Alternate Law 1 (ALT1) combines Normal Law lateral mode with Alternate Law pitch modes. Low Energy Protection is replaced by Low Speed Stability meaning that the aircraft no longer has automatic stall protection. At low speed, a nose down demand is introduced based on IAS (instead of AOA) and Alternate Law changes to Direct Law. In addition, an audio "STALL" warning is introduced. α-Floor protection is not available so conventional pilot stall recovery action is required.

Load Factor and Bank Angle Protections are retained. High Speed and High Angle of Attack Protections enter Alternate Law mode. Pitch Attitude Protection is lost.

ALT1 control law degradation will result from some faults in the horizontal stabilizer, a single elevator fault, loss of a yaw-damper actuator, loss of slat or flap position sensors or a single air data reference fault. Dependent upon the failure, autopilot may not be available.

In Alternate Law 2 (ALT2), Normal Law lateral mode is lost and is replaced by roll Direct Law and yaw Alternate Law. Pitch mode is in Alternate Law. Load factor protection is retained. In addition to those protections lost in ALT1 (Pitch Attitude and Low Energy Protection), Bank Angle Protection is also lost. In some failure cases, High Angle of Attack and High Speed Protections will also be lost.

As is the case with ALT1, some failure cases that result in ALT2 will also cause the autopilot to disconnect. ALT2 is entered when both engines flame out, with faults in two inertial or two air-data reference units, with faults to all spoilers, certain aileron faults or with a pedal transducers fault

Direct law
In Direct Law (DIR), lateral modes are the same as ALT2; that is roll Direct Law and yaw Alternate Law. Pitch control degrades to Direct Law and automatic trim is inoperative requiring stab trim to be adjusted manually by the pilot. Control surface motion is directly related to the sidestick motion. ALL protections are lost.

In Direct Law, autopilot function is always lost. DIR is entered if there is failure of all three inertial reference units or all three primary flight computers, faults in both elevators or flame out of both engines concurrent with loss of PRIM 1.

Boeing uses:
Normal mode
In Normal mode during manual flight, the ACEs receive pilot control inputs and send these signals to the three PFCs. The PFCs verify these signals and utilise information from other airplane systems in order to compute control surface commands. These commands are then sent back to the ACEs which then send the enhanced signals to the flight control surface actuators which convert them into analog servo commands. Full functionality is provided including all enhanced performance, envelope protection and ride quality features.

When the autopilot is engaged, the autopilot system sends commands to the PFCs. The PFCs generate control surface commands which are sent to the ACEs in the same manner as pilot control inputs. The autopilot commands move the flight deck controls to provide autopilot feedback to the pilots. If a pilot overrides the autopilot with control inputs, the PFCs will disengage the autopilot and utilise the pilot control inputs. Note that the autopilot is not available should reversion to Secondary or Direct mode occur.

Secondary mode
Boeing Secondary mode is somewhat similar to the Airbus Alternate Law. When the PFCs cannot support Normal mode operation due to internal faults or to loss of information from other aircraft systems, they automatically revert to Secondary mode. Reversion to Secondary mode results in the loss of the autopilot and the pilots must control the aircraft manually. The ACEs still receive pilot control inputs and send the appropriate signals to the PFCs. However, due to the degraded mode of operation, the PFCs use "simplified" computations to generate the flight control surface commands. These commands are sent back to the ACEs from where they are sent to the flight control surfaces in the same manner as during Normal mode operations.

Aircraft handling qualities are affected by the simplified computations or PFC control laws that are utilised in Secondary mode. While all flight control surfaces remain operative, the elevator and rudder are more sensitive at some airspeeds. The following functions are inoperative or degraded during Secondary mode operations:

• autopilot
• auto speedbrakes
• envelope protection
• gust suppression
• tail strike protection
• thrust asymmetry compensation
• yaw damping

Direct mode
The ACEs automatically revert to Direct mode when they detect the failure of all three PFCs or when they are unable to communicate with the PFCs. Direct mode can also be manually selected by selecting the DISC position on the Primary Flight Computers Disconnect switch. In Direct mode, the PFCs no longer generate control surface commands. Pilot inputs are received by the ACEs and sent directly to the flight control surface actuators.

Direct mode allows for full aircraft control while in flight and during the landing phase. Aircraft handling characteristics are very similar to those encountered while in Secondary mode. In addition to those functions lost during Secondary mode operations (as listed previously) the manual rudder trim cancel switch is inoperative.