A key for use in a cylinder lock with a rotatable key plug having a profiled key way, said key comprising: - an elongated, substantially flat key blade having a longitudinal profile groove extending along at least a portion of the length of the key blade, with an inner wall of said groove being substantially parallel to a side surface of said flat key blade, - said longitudinal profile groove having an undercut portion adjacent to and inside a ridge portion of the key blade, the outside of said ridge portion forming a part of said side surface of the key blade, characterized in that - said undercut portion of said longitudinal profile groove, at its innermost part inside said ridge portion, is expanded into a longitudinal pocket having opposite lateral wall portions (132,133) and a lowermost transverse end wall, which is substantially flat or slightly curved; - said pocket forms an extension of said undercut portion of the profile groove, said extension being oriented substantially in a direction towards an edge portion of the key blade, said direction being parallel to said side surface of the key blade; - said lowermost transverse end wall is substantially parallel to a lower edge portion of the key blade and faces upwardly in a direction in a central plane of the key blade; - said ridge portion forms a massive material portion extending in parallel to the side surface of the key blade, and - the vertical dimension (h) of said ridge portion, measured in a plane of said side surface of the key blade, is more than half of the smallest width (w) of said undercut groove adjacent to said side surface, said smallest width being measured as a perpendicular projection onto said inner wall of said longitudinal groove. - an elongated, substantially flat key blade having a longitudinal profile groove extending along at least a portion of the length of the key blade, with an inner wall of said groove being substantially parallel to a side surface of said flat key blade, - said longitudinal profile groove having an undercut portion adjacent to and inside a ridge portion of the key blade, the outside of said ridge portion forming a part of said side surface of the key blade, - said undercut portion of said longitudinal profile groove, at its innermost part inside said ridge portion, is expanded into a longitudinal pocket having opposite lateral wall portions (132,133) and a lowermost transverse end wall, which is substantially flat or slightly curved; - said pocket forms an extension of said undercut portion of the profile groove, said extension being oriented substantially in a direction towards an edge portion of the key blade, said direction being parallel to said side surface of the key blade; - said lowermost transverse end wall is substantially parallel to a lower edge portion of the key blade and faces upwardly in a direction in a central plane of the key blade; - said ridge portion forms a massive material portion extending in parallel to the side surface of the key blade, and - the vertical dimension (h) of said ridge portion, measured in a plane of said side surface of the key blade, is more than half of the smallest width (w) of said undercut groove adjacent to said side surface, said smallest width being measured as a perpendicular projection onto said inner wall of said longitudinal groove.