Retention device for retaining blanks comprising: - a support, - a first gripping portion which is secured to said support and which is provided with first retention elements of said blank, - a second gripping portion which is secured to said support or to said first gripping portion and which is provided with second retention elements of said blank, - a movement mechanism for said second gripping portion with respect to said first gripping portion, which movement mechanism is configured to carry out a pure rotation of said second gripping portion about a virtual rotation axis which is located outside said retention device, - wherein said movement mechanism comprises an articulated parallelogram which is secured to said first gripping portion and said second gripping portion, - characterised in that said articulated parallelogram comprises - a first rod which is secured by means of a first hinge to a first rotation point of said first gripping portion, - a second rod which is secured by means of a second hinge to a second rotation point of said first gripping portion, - a third rod comprising a first pin which is secured with permitted rotation to said first rod, a second pin which is secured with permitted rotation to said second rod and a third pin which is secured with permitted rotation to a third rotation point of said second gripping portion, - a fourth rod comprising a fourth pin which is secured with permitted rotation to said first rod, a fifth pin which is secured with permitted rotation to said second rod and a third pin which is secured with permitted rotation to a fourth rotation point of said second gripping portion, - said first and second rotation points (11, 12) and said virtual rotation axis being aligned with each other, - said third and fourth rotation points (21, 22) and said virtual rotation axis being aligned with each other, - in such a manner that, when said first and second rods (60, 70) are rotated with respect to said first and second rotation points (11, 12) through the same angle (α), said third and fourth rods (80, 90) follow a translational movement with respect to said first gripping portion while maintaining a mutual parallelism condition, thereby varying the angular orientation (β) of said second gripping portion with respect to said first gripping portion by rotating about said virtual rotation axis. - a support, - a first gripping portion which is secured to said support and which is provided with first retention elements of said blank, - a second gripping portion which is secured to said support or to said first gripping portion and which is provided with second retention elements of said blank, - a movement mechanism for said second gripping portion with respect to said first gripping portion, which movement mechanism is configured to carry out a pure rotation of said second gripping portion about a virtual rotation axis which is located outside said retention device, - wherein said movement mechanism comprises an articulated parallelogram which is secured to said first gripping portion and said second gripping portion, - characterised in that said articulated parallelogram comprises - a first rod which is secured by means of a first hinge to a first rotation point of said first gripping portion, - a second rod which is secured by means of a second hinge to a second rotation point of said first gripping portion, - a third rod comprising a first pin which is secured with permitted rotation to said first rod, a second pin which is secured with permitted rotation to said second rod and a third pin which is secured with permitted rotation to a third rotation point of said second gripping portion, - a fourth rod comprising a fourth pin which is secured with permitted rotation to said first rod, a fifth pin which is secured with permitted rotation to said second rod and a third pin which is secured with permitted rotation to a fourth rotation point of said second gripping portion, - said first and second rotation points (11, 12) and said virtual rotation axis being aligned with each other, - said third and fourth rotation points (21, 22) and said virtual rotation axis being aligned with each other, - in such a manner that, when said first and second rods (60, 70) are rotated with respect to said first and second rotation points (11, 12) through the same angle (α), said third and fourth rods (80, 90) follow a translational movement with respect to said first gripping portion while maintaining a mutual parallelism condition, thereby varying the angular orientation (β) of said second gripping portion with respect to said first gripping portion by rotating about said virtual rotation axis.