榫眼

榫眼	sǔn yǎn	mortise (slot cut into wood to receive a tenon)
榫	sǔn	(cabinetmaking) tenon
眼	yǎn	eye (CL:只,双) / (often used with 一) a look; a glance / small hole / (bound form) salient point / classifier for wells, cave-dwellings etc