A user interface comprising: a. a mask body and a sealing cushion connected to the mask body and configured to seal around a patient's nose and/or mouth; wherein the mask body and sealing cushion together define an interior chamber of the user interface; b. a breathing gas delivery inlet configured to receive a breathing gas flow from a gas source; and c. at least one non-sealing nasal prong in the interior chamber; characterized in that: the user interface further comprises a breathing gas flow director configured to split the breathing gas flow into two flow paths (F2, F3), such that a portion of the breathing gas flow is delivered along a first flow path from the inlet into the interior chamber to pressurise the interior chamber, and such that a portion of the breathing gas flow is delivered along a second flow path from the inlet through the at least one non-sealing nasal prong, wherein the at least one non-sealing nasal prong comprises part of a nasal prong assembly mounted in the mask body, the nasal prong assembly comprising a manifold from which the nasal prong projects, and wherein the user interface also comprises at least one laterally extending arm which extends from the manifold through the interior chamber such that a distal end of the arm projects externally of the mask body, the distal end comprising a vent outlet, the arm defining a vent passage through a portion of the arm, the vent passage being in fluid communication with the interior chamber and the vent outlet. a. a mask body and a sealing cushion connected to the mask body and configured to seal around a patient's nose and/or mouth; wherein the mask body and sealing cushion together define an interior chamber of the user interface; b. a breathing gas delivery inlet configured to receive a breathing gas flow from a gas source; and c. at least one non-sealing nasal prong in the interior chamber; characterized in that: the user interface further comprises a breathing gas flow director configured to split the breathing gas flow into two flow paths (F2, F3), such that a portion of the breathing gas flow is delivered along a first flow path from the inlet into the interior chamber to pressurise the interior chamber, and such that a portion of the breathing gas flow is delivered along a second flow path from the inlet through the at least one non-sealing nasal prong, wherein the at least one non-sealing nasal prong comprises part of a nasal prong assembly mounted in the mask body, the nasal prong assembly comprising a manifold from which the nasal prong projects, and wherein the user interface also comprises at least one laterally extending arm which extends from the manifold through the interior chamber such that a distal end of the arm projects externally of the mask body, the distal end comprising a vent outlet, the arm defining a vent passage through a portion of the arm, the vent passage being in fluid communication with the interior chamber and the vent outlet. the user interface further comprises a breathing gas flow director configured to split the breathing gas flow into two flow paths (F2, F3), such that a portion of the breathing gas flow is delivered along a first flow path from the inlet into the interior chamber to pressurise the interior chamber, and such that a portion of the breathing gas flow is delivered along a second flow path from the inlet through the at least one non-sealing nasal prong, wherein the at least one non-sealing nasal prong comprises part of a nasal prong assembly mounted in the mask body, the nasal prong assembly comprising a manifold from which the nasal prong projects, and