Spiral steel bones are the most flexible bones and enhance curves. Their flexibility makes them a great choice for dancewear, but they are not a good choice if you need a lot of support. 
Spiral bones are made of steel, looking as if a fine coil of steel has been flattened. Spiral steel bones are available in four sizes, 3/16" (4mm), 1/4" (6mm), 7mm (not much wider than the 6mm but stronger because it's thicker steel) and 7/16"(11 mm). The 1/4" comes either by the metre or in pre-tipped lengths (both in galvanized wire) and the 7/16" is sold by the metre in a grey galvanized colour and by the piece with a shiny zinc coating. Spiral bones have the most flexibility of all bones, being able to move both forwards and backwards and side to side. They are the best choice for a bodice that requires boning but is going to be worn for dance or other motion activities. They are not a good choice for front panels of bodices/corsets for women who wear a c-cup or more, unless you are using the 7/16" size. Spiral bones are the most flexible and least supportive. Bodices with these can be dry cleaned but not laundered with water.