Aurifil Says this about their 50wt thread.

The 50wt is strong and thin, and results in remarkably flat and crisp seams. While it is wonderful for piecing, subtle quilting & topstitching, and English Paper Piecing it also works incredibly well for Needle Turn Applique, Bobbin and Machine Lace, Machine Applique (Straight Stitch, ZigZag, Blind Hem and Blanket Stitch,) Machine Embroidery, Dense Machine Quilting, Subtle Machine and Longarm Quilting, Basting, Whole Cloth and Micro Quilting, and Dense Background Designs.

The spool is 1300 metres or 1422 yards