A rack system includes first and second slide rail assemblies mounted respectively on first and second posts of a rack. The first and second posts define a width therebetween. Each slide rail assembly includes a first rail with a first channel, a second rail displaceable with respect to the first rail, and a third rail movably provided between the first channel of the first rail and the second rail. The first rail of each slide rail assembly has a rail-thickness-wise cross section outside the space in which the width defined by the rack extends, whereas the second and third rails of each slide rail assembly are within the space in which the width defined by the rack extends.