A tensioning device is described for mounting between a knotter and a twine supply roll in a baler. The device comprises a spring biased tensioning arm 210 for taking up slack in the twine 216 and a clamp 220 for gripping the twine 216 at a location between the tensioning arm 210 and the supply roll. In the invention, the clamp 220 is coupled to the tensioning arm 210 in such a manner that the force applied by the clamp 220 to grip the twine 216 varies with the position of the tensioning arm 210.