A fenestration (32) for a stent graft (30). The fenestration is an aperture in the biocompatible graft material and has at least one flap (38, 40) of a biocompatible graft material covering the aperture on the inside whereby the flap closes off the aperture but can be displaced to allow access through the fenestration. An array of such fenestrations may be placed on a stent graft to facilitate alignment of a branch vessel with a fenestration. A slip knot (46, 46) which can be released by forcing a dilator between the flaps can be used to hold the flaps together.