Underlay - Underlay is stitching underneath the actual embroidery stitching. It raises the embroidery stitching above the surface of the fabric, giving depth and texture to the embroidery. It eliminates the need for high density in small areas of embroidery. Density - Density is a term that means the distance between two stitches. If density is low, there is more space between the stitches than if density is high.
Density is the single most important factor in making embroidery look good.
Too little density creates stitches that do not cover the fabric. Too much density creates stitches that bunch up, create lumps, is hard, and breaks thread and needles. The larger the embroidery, the higher the density must be to provide adequate coverage. Density can be lower in small areas of embroidery. Firmer fabrics will require higher density than lighter or knit fabrics.