Connecting device comprising a first line end, a second line end and a connecting ring, by means of which the first line end can be connected to the second line end, wherein - the first line end has ∘ a flange-like radial first widening with a first end face and a first rear face, or ∘ a plurality of flange-like radial first widenings, each with a first end face and a first rear face, wherein the first widenings are separated from one another by interruption portions, and - the second line end has a plurality of flange-like radial second widenings, each with a second end face and a second rear face, wherein the second widenings are separated from one another by interruption portions, wherein - the connecting ring is furnished with ∘ a first closure face or a plurality of first closure faces and ∘ a plurality of second closure faces, wherein the plurality of first closure faces and the plurality of second closure faces are separated from one another by passage portions, wherein - the first closure face can be brought into contact with the first rear face, or the first closure faces can be brought into contact with the first rear faces, and - the second closure faces can be brought into contact with the second rear faces by virtue of the fact that the second widenings are guided with an axial movement through the passage portions and then a relative rotational movement is carried out between the connecting ring and the first line end and/or a relative rotational movement is carried out between the connecting ring and the second line end, wherein a sealing element is attached to the first line end or the second line end, characterised in that the first rear face and the second rear face are inclined with respect to the first end face or the second end face, in that the first line end and the second line end are configured as a standard tri-clamp fitting and comprise a first inclined surface or a second inclined surface respectively, and in that the connecting ring is configured to complement the line ends (12, 14) connected to one another and configured as standard tri-clamp fittings. - the first line end has ∘ a flange-like radial first widening with a first end face and a first rear face, or ∘ a plurality of flange-like radial first widenings, each with a first end face and a first rear face, wherein the first widenings are separated from one another by interruption portions, and ∘ a flange-like radial first widening with a first end face and a first rear face, or ∘ a plurality of flange-like radial first widenings, each with a first end face and a first rear face, wherein the first widenings are separated from one another by interruption portions, and - the second line end has a plurality of flange-like radial second widenings, each with a second end face and a second rear face, wherein the second widenings are separated from one another by interruption portions, wherein - the connecting ring is furnished with ∘ a first closure face or a plurality of first closure faces and ∘ a plurality of second closure faces, wherein the plurality of first closure faces and the plurality of second closure faces are separated from one another by passage portions, wherein ∘ a first closure face or a plurality of first closure faces and ∘ a plurality of second closure faces, wherein the plurality of first closure faces and the plurality of second closure faces are separated from one another by passage portions, wherein - the first closure face can be brought into contact with the first rear face, or the first closure faces can be brought into contact with the first rear faces, and - the second closure faces can be brought into contact with the second rear faces by virtue of the fact that the second widenings are guided with an axial movement through the passage portions and then a relative rotational movement is carried out between the connecting ring and the first line end and/or a relative rotational movement is carried out between the connecting ring and the second line end, wherein a sealing element is attached to the first line end or the second line end,