"In quilting, foundation piecing was originally used to stabilise pieces of fabric that were stitched together. It first became popular in the 18th and 19th centuries in England.
Originally, pieces of scrap, fabric, or muslin were used as the foundation. Now, the use of paper, whether tracing paper, freezer paper or some other heavy weight paper, has become very popular for the use as a pattern in creating quilt blocks that are all the same size, each with precise, sharp points and perfectly matched intersections. In addition, information such as color and fabric choices can be written on the paper foundation in order to facilitate the construction of the piece and leaving lesser room for error while sewing.
With paper piecing, a shorter stitch length (e.g. 12 stitches per inch) is used when sewing the components together, which makes it easier to tear away the foundation after the block is completed. Several methods can be used to prepare the design for use on the foundation: tracing, photocopying, computer printing, needle punching, and pre-set designs." (Wikipedia)