Coupling (10, 110), comprising a receiving element (12, 112), a securing element (14, 114), at least one retaining element (16, 116) formed separately from the securing element (14, 114), and an insertion element (18, 118), wherein the insertion element (18, 118) is insertable into a receiving space of the receiving element (12, 112) along an insertion axis, wherein the securing element (14, 114) is adapted to be displaceable along a displacement axis between a release position and a locking position, wherein the securing element (14, 114) is adapted to engage with the at least one retaining element (16, 116) in its release position, without an insertion element (18, 118) inserted into the receiving space of the receiving element (12, 112), in such a way that a displacement of the securing element (14, 114) from the release position in a direction of the locking position is prevented, wherein the insertion element (18, 118) is adapted to interact with the at least one retaining element (16, 116) when the insertion element (18, 118) is inserted into the receiving space of the receiving element (12, 112), in such a way that a displacement of the securing element (14, 114) from the release position into the locking position is enabled, wherein each retaining element (16, 116) includes a retaining arm (24, 124) extending parallel to the insertion axis, the retaining arm being resilient in a radial direction with respect to the insertion axis, and wherein each retaining element (16, 116) has a projection (26, 126) extending radially inwards when connected to the receiving element (12, 112), characterised in that a portion of the projection (26, 126) extending furthest radially inwards extends further radially inwards than a portion extending furthest radially inwards of an inner wall of the receiving space into which the projection (26, 126) extends, but is disposed further radially outwards than a portion extending furthest radially inwards of an inner wall of a through-hole of the securing element (14, 114) through which the insertion element (18, 118) is passed when the insertion element (18, 118) is connected to the receiving element (12, 112). a receiving element (12, 112), a securing element (14, 114), at least one retaining element (16, 116) formed separately from the securing element (14, 114), and an insertion element (18, 118), wherein the insertion element (18, 118) is insertable into a receiving space of the receiving element (12, 112) along an insertion axis, wherein the securing element (14, 114) is adapted to be displaceable along a displacement axis between a release position and a locking position, wherein the securing element (14, 114) is adapted to engage with the at least one retaining element (16, 116) in its release position, without an insertion element (18, 118) inserted into the receiving space of the receiving element (12, 112), in such a way that a displacement of the securing element (14, 114) from the release position in a direction of the locking position is prevented, wherein the insertion element (18, 118) is adapted to interact with the at least one retaining element (16, 116) when the insertion element (18, 118) is inserted into the receiving space of the receiving element (12, 112), in such a way that a displacement of the securing element (14, 114) from the release position into the locking position is enabled, wherein each retaining element (16, 116) includes a retaining arm (24, 124) extending parallel to the insertion axis, the retaining arm being resilient in a radial direction with respect to the insertion axis, and wherein each retaining element (16, 116) has a projection (26, 126) extending radially inwards when connected to the receiving element (12, 112), characterised in that a portion of the projection (26, 126) extending furthest radially inwards extends further radially inwards than a portion extending furthest radially inwards of an inner wall of the receiving space into which the projection (26, 126) extends, but is disposed further radially outwards than a portion extending furthest radially inwards of an inner wall of a through-hole of the securing element (14, 114) through which the insertion element (18, 118) is passed when the insertion element (18, 118) is connected to the receiving element (12, 112).