Helix 30cm300mm folding rule is the pencil case essential this uniquely developed folding rule flips open to twice its size for longer measurements with a plastic snap fit hinge for easy compact storage ideal for young children students or office use this rule has both cm and mm measurements with clear transparent plastic for perfect alignment the shatterproof rule will withstand any accidents and the click mechanism will stop the ruler unfolding to keep it neat and secure in your pencil ca