What's the difference between the Sulky 30 weight and 12 weight Cotton Threads?

The heavy 12 wt. Cotton is a dramatic looking top thread in both the 66 Solid Colors and the 84 Cotton Blendables. We generally recommend using the 12 wt. on the top (although it is used for bobbin-work as well), and the matching 30 wt. in the bobbin.