A socket for rotatably fastening or unfastening a jackbolt of a multi jackbolt tensioner, the jackbolt including a jackbolt head (H) having a plurality of sides defining a plurality of corners (C), the socket extending between a first end and a second end such that the first end is adapted to receive the jackbolt, the socket comprising: a passage extending along a length of the socket to receive the jackbolt head (H), the passage being defined by a plurality of even-numbered internal side walls dimensioned to engage the plurality of sides of the jackbolt head (H) received therein, the socket having a central axis (X) with diametrically opposed pairs of said side walls being parallel to each other, and pairs of adjacent side walls meeting to form corner portions to receive corners (C) of the jackbolt head (H); a plurality of guiding members arranged circumferentially about an opening at the first end of the socket to guide the jackbolt head (H) into the passage and align the plurality of sides of the jackbolt head (H) such that corners (C) of the jackbolt head (H) are aligned into position with corner portions of said pairs of adjacent side walls , characterised in that each guiding member comprises a pair of guiding surfaces wherein the guiding surfaces for each pair are angled relative to each other such that upon contacting the jackbolt head (H), a side of the jackbolt head (H) is biased by at least one of the guiding surfaces of at least one of the plurality of guiding members to align the corners (C) of the jackbolt head (H) into position with the corner portions of said pairs of adjacent side walls in the passage , wherein the guiding surfaces of the circumferentially arranged guiding members slope inwardly and downwardly from a circumferential outer edge portion of the socket towards a radially inner portion of the socket that is axially offset from the circumferential outer portion, and wherein each of the guiding surfaces in said pair of guiding surfaces diverges away from each other towards the circumferential outer edge. a passage extending along a length of the socket to receive the jackbolt head (H), the passage being defined by a plurality of even-numbered internal side walls dimensioned to engage the plurality of sides of the jackbolt head (H) received therein, the socket having a central axis (X) with diametrically opposed pairs of said side walls being parallel to each other, and pairs of adjacent side walls meeting to form corner portions to receive corners (C) of the jackbolt head (H); a plurality of guiding members arranged circumferentially about an opening at the first end of the socket to guide the jackbolt head (H) into the passage and align the plurality of sides of the jackbolt head (H) such that corners (C) of the jackbolt head (H) are aligned into position with corner portions of said pairs of adjacent side walls , characterised in that each guiding member comprises a pair of guiding surfaces wherein the guiding surfaces for each pair are angled relative to each other such that upon contacting the jackbolt head (H), a side of the jackbolt head (H) is biased by at least one of the guiding surfaces of at least one of the plurality of guiding members to align the corners (C) of the jackbolt head (H) into position with the corner portions of said pairs of adjacent side walls in the passage , wherein the guiding surfaces of the circumferentially arranged guiding members slope inwardly and downwardly from a circumferential outer edge portion of the socket towards a radially inner portion of the socket that is axially offset from the circumferential outer portion, and wherein each of the guiding surfaces in said pair of guiding surfaces diverges away from each other towards the circumferential outer edge. <EOS>