Machine embroidery thread generally comes in three sizes: 50wt., 40 wt., and 30 wt. The lower the number, the heavier the thread. Most embroidery designs that are digitized for CDs, cards, etc. are designed to be used with 40 wt. thread, unless specifically stated otherwise. A 30 wt. thread may result in a design that is too dense. Conversely, a 50 wt. would leave a sparse design. That said, don’t be afraid to experiment; many people use different thread weights to give their designs a unique look.