A set of racks for mounting sub-racks for optical, electronic and/or electrical equipment, the set comprising a first rack and two second racks (810, 820), wherein - the first rack comprises a first rack bottom, a first rack top, a first left side rack up-right and a first right side rack up-right; - the first left side rack up-right and the first right side rack up-right each comprise a front sub-rack mounting flange (801-1, 802-1) and a rear sub-rack mounting flange (801-2, 802-2), each for mounting a sub-rack; - each second rack (810, 820) comprises a second rack bottom, a second rack top, a second left side rack up-right (811, 821) and a second right side rack up-right (812, 822); - the second left side rack up-right (811, 821) and the second right side rack up-right (812, 822) of each second rack (810, 820) comprise a front sub-rack mounting flange (811-1, 821-1, 812-1, 822-1) for mounting a sub-rack; - the second rack bottom, the second rack top, the second left side rack up-right (811, 821) and the second right side rack up-right (812, 822) of each second rack (810, 820) are firmly attached to one another, such that they form a supporting structure for the same second rack (810, 820); characterized in that - the front sub-rack mounting flanges (811-1, 812-1,) of one of the second racks (810,) are aligned with the front sub-rack mounting flanges (801-1, 802-1) of the first rack; - the front sub-rack mounting flanges (821-1, 822-1) of the other of the second racks are aligned with the rear sub-rack mounting flanges (801-2, 802-2) of the first rack; and - the two second racks (810, 820) are mounted back-to-back to have a combined depth which is the same as a depth of the first rack. - the first rack comprises a first rack bottom, a first rack top, a first left side rack up-right and a first right side rack up-right; - the first left side rack up-right and the first right side rack up-right each comprise a front sub-rack mounting flange (801-1, 802-1) and a rear sub-rack mounting flange (801-2, 802-2), each for mounting a sub-rack; - each second rack (810, 820) comprises a second rack bottom, a second rack top, a second left side rack up-right (811, 821) and a second right side rack up-right (812, 822); - the second left side rack up-right (811, 821) and the second right side rack up-right (812, 822) of each second rack (810, 820) comprise a front sub-rack mounting flange (811-1, 821-1, 812-1, 822-1) for mounting a sub-rack; - the second rack bottom, the second rack top, the second left side rack up-right (811, 821) and the second right side rack up-right (812, 822) of each second rack (810, 820) are firmly attached to one another, such that they form a supporting structure for the same second rack (810, 820); characterized in that - the front sub-rack mounting flanges (811-1, 812-1,) of one of the second racks (810,) are aligned with the front sub-rack mounting flanges (801-1, 802-1) of the first rack; - the front sub-rack mounting flanges (821-1, 822-1) of the other of the second racks are aligned with the rear sub-rack mounting flanges (801-2, 802-2) of the first rack; and - the two second racks (810, 820) are mounted back-to-back to have a combined depth which is the same as a depth of the first rack.