Abstract: A multi-turn coil structure is wound on a former starting from a central portion of the coil and spirally winding respective lengths of the coil which extend from the central portion around the former in opposite winding directions and in adjacent contiguous planes. This allows outer end portions of the respective windings which constitute input and output leads for the coil each to be accommodated within the bi-planar coil structure. This reduces the overall thickness of the coil compared with conventional structures in which an inner end of the coil structure is extended to the outside in a further contiguous plane adjacent the coil. The structure is particularly suited for miniature wire coils to be used in electronic circuits of security cards.