Definition:
Congress cloth is a fine-meshed, 24 holes-per-inch even weave canvas. It is used primarily for Petit Point or miniature needlepoint.
Very fine, thin threads are used to stitch a needlepoint design on Congress Cloth, including single plies of 6-strand floss, fine silk, rayon or wool threads.
In the photo I have placed a small paperclip on the canvas to show the scale.