Good quality ready to wear garments will often have an invisible zip fitted. These zips are often neglected by the home dressmaker which is a pity because they can ensure success everytime. Invisible zips are suitable for all fabrics from fine silk jersey through to suit weight wools and tweeds. The key is to get the needle as close to the fastener as possible, which this foot accomplishes by slightly unrolling the zip just before the needle. Once fitted, a well sewn invisible zip will merely look like part of the seam.

Angela says:-

A concealed or invisible zip gives a very clean finish as it is stitched directly into the seam. When the zip is closed, the tab or pull is the only part visible.

The most important thing to remember is to sew the zip before sewing the seam.

Always buy a concealed zip longer than the required length as you will not use the entire length of the zip. Use this type of zip on velvet as topstitching would crush the velvet pile and be a disaster.