In order to check the authenticity of a banknote or other such document it is printed with two patches of magnetisable ink, and each patch is magnetised to present a multipole sequence of alternating polarity. If the document is folded to bring the two patches together and then rubbed to and fro in the direction of the pole sequences, they will be subject to alternating forces of attraction and repulsion which can be sensed through the fingertips and gives the impression of a physically rippled texture notwithstanding that the patches actually have a smooth surface. The presence or absence of this effect can therefore be used to distinguish between a genuine document bearing such magnetised patches and a counterfeit which may be visually identical but lacks the correct magnetisation. In a variant only one of the patches is printed on the document itself and the other is on a separate "key" device which is rubbed over it to test for the presence of the correct magnetisation.