From Brewer Sewing:
These top quality wood hoops were especially designed for machine embroidery - they are thinner and easily slide under any machine needle. Because they are thinner, they are super for silk ribbon embroidery when it is important for your fingers to reach the center of the hoop to help control the ribbon. Adjustable brass screws allow you to tightly secure fabrics. These are the finest hoops made and will last a lifetime.