A teatcup liner comprises an upper mouthpiece, an intermediate barrel and a lower connecting tube. The upper mouthpiece further comprises a mouthpiece transition, a mouthpiece lip, and a groove formed in an outer surface of the upper mouthpiece. The intermediate barrel extends along an axial direction for receiving a teat inserted axially thereinto through said mouthpiece. An inner dimension of the upper mouthpiece defines a cavity bore and the groove is located substantially in line with the largest diameter of the cavity bore.