Basting Stitch is the longest stitch on the machine. Speed basting is a feature found on many machines, making stitches up to 1" long or more. Check your manual. Speed basting can be used for all basting purposes, but should be tested first on your fabric: Some machines make "empty" stitches between the long ones, and the line of needle holes might show. Hold the fabric taut in front and back, because the basting has a tendency to bunch up the fabric behind the needle.
In Ease-Stitching and Gathering, the line of stitching is drawn up to give either ease or fullness.
Understitching is a line of stitching done on a facing or the underside of a collar. Made close to the edge to prevent seams from rolling to the outside, it does not show.
Stitching in the Ditch means stitching on the right side, in pressed-open seam, to fasten another piece of fabric underneath. It is here shown on a neck facing (26, 27). Done with matching thread and regular stitch length, it is invisible on the outside because it disappears into the "ditch" of the seam. Draw thread-ends through to wrong side, tie and clip.
Stay-Stitching is a line of directional stitching, made before construction, to stabilize edges that might be pulled out of shape in handling. It can safely be omitted on fabrics that are firmly woven or tightly knitted. It is a must, however, on off-grain in loosely-woven fabrics, also on bias edges and knits where the stretch needs control. On curved edges that will be clipped later, stay-stitching helps prevent the clipping from weakening the seam.
Stay-stitching is done on the separate garment pieces, the first thing after cutting and marking, through a single thickness. When an interfacing or an underling is added, however, the stitching that attaches it serves as stay-stitching, and is done directionally where required.
The diagram shows where stay-stitching is usually needed, and the arrow show how to do it directionally. Using regular stitch length, stitch ½" from the raw edge (to keep clear of the seam line). At a zipper opening, stay-stitch ¼" from raw edge.
from: COATS & CLARK; Sewing A TO Z