(57) Improved sheet material of the type wherein a base sheet is coated on at least one face with a layer comprising a film-forming polymer containing inter-connected microvoids, the layer being locally transparentizable to display contrasting indicia when there is applied to its exposed surface a transparent, colorless liquid which has a refractive index similar to that of the solid constituents of the layer. A substance which jellifies the liquid is incorporated in the layer, whereby the indicia maintain substantially the same dimensions throughout the time that the layer is locally transparentized.