Abstract: A fastening device, or clamp, for attaching margins of a pair of flexible sheets is provided. The clamp comprises a first strand and a second strand extending along the margins of the sheets. The first and second strands are periodically resiliently biased toward one another to clamp the overlapping margins of the sheets therebetween. The first strand can be straight, while the second strand can undulate. The first strand and the second strand can comprise metallic wires. In one embodiment, the clamp comprises a gripping member that is a resilient C-shaped clamp configured to snap flexible sheets about a substantially straight strand. The clamp preferably is used to attach adjacent flexible sheets to a structural element of a greenhouse.