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.  The 1/4" comes either by the metre or in pre-tipped lengths (both in galvanized wire). 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. Bodices with these can be dry cleaned but not laundered with water.

Sprial Boning 1/4"- 6 mm