The previous definition of a linear system is applicable to SISO (single-input single-output) systems. For MIMO (multiple-input multiple-output) systems, input and output signal vectors ( x 1 ( t ) , x 2 ( t ) , y 1 ( t ) , y 2 ( t ) ) are considered instead of input and output signals ( x 1 ( t ) , x 2 ( t ) , y 1 ( t ) , y 2 ( t ) .)