Modular construction system with rotating coupling

A modular construction system, comprising at least two profile parts for mutual releasable coupling by means of a coupling part, wherein the coupling part is provided on at least two different sides thereof with one or more radially protruding lips and a reduced radial section adjacent to and coplanar with the protruding lips, wherein the reduced radial section further includes a recessed portion present behind the lips as seen in coupling direction, and wherein each profile part is provided in at least one wall thereof with a continuous opening with different dimensions as seen in at least two different radial directions, wherein the larger dimension is larger than or equal to the maximum radial section through the lips and wherein the smaller dimension is smaller than said maximum radial section but larger or equal to said reduced radial section, wherein the wall thickness at the position of the opening is smaller than or equal to the width of the recessed portion on the coupling part, such that coupling takes place by freely positioning of the lips in the opening and by subsequent radial rotation of the coupling part and each profile part relative to each other to engage the lips against, at least one wall of the profile part thereby securing the coupling part and one profile part together.