Apparatus and methods for use in the manufacture of a spring unit for incorporation into an upholstered article, for example, a mattress, cushion or the like. A coil interlinking process comprises compressing a first coil to define a clearance extending a second coil passed the first coil via the clearance, allowing the first coil to extend across the clearance, and contracting the second coil such that the second coil engages the first coil thereby interlining the first and second coils. The invention also relates to a coil interlinking apparatus.