A receiving part for connecting a shank of a bone anchoring element to a rod. The receiving part has a first end, a second end and a recess having a substantially U-shaped cross section. The recess extends from the first end in a direction of the second end forming two legs open at the first end. The recess defines a channel to receive the rod. The channel has a longitudinal axis. The legs have an inner thread to receive a securing element having an outer thread cooperating with the inner thread. The legs are provided with stiffening structures. In one embodiment, the stiffening structures are formed by the wall thickness of the outer edges of the legs being larger than the wall thickness of the legs at a location closest to the central axis of the receiving part.