Underlining

Underlining (sometimes called mounting or backing) is a second layer of fabric cut to the same shape as each garment piece and attached to its wrong side before any sewing starts. From then on, the fashion fabric and underlining are handled as a single layer: darts, seams and hems are all sewn through both. Unlike a lining, it isn't a separate garment, and unlike interfacing, it covers the whole piece.

Underlining, lining and interfacing

• Lining is a separate inner garment, assembled on its own and attached only at the edges (neckline, armholes, waist, hem). It hides the inside construction, helps the garment slide on, and can be seen when you look inside. See Lining Fabrics.
• Underlining is attached to each piece and sewn in with it. It changes how the fashion fabric behaves and is hidden inside the seams.
• Interfacing supports only selected areas, such as collars, facings, waistbands and buttonhole bands, to make them firmer. See Fusible Interfacing.

A garment can have all three, and couture garments often do.

What underlining does

• Adds opacity to fabrics that are slightly see-through, such as pale linen, lace or a thin white cotton, without the look of a separate lining.
• Adds body so a soft fabric holds a structured shape. A silk dupion or charmeuse skirt underlined with organza holds a crisp A-line.
• Supports loosely woven or delicate fabrics, taking the strain off seams so they don't pull apart.
• Gives you somewhere to sew invisible hand stitches. Hems, facings and seam allowances can be catch-stitched to the underlining, so nothing shows on the outside.
• Covers construction markings. Darts and seamlines can be marked on the underlining instead of the fashion fabric.
• Reduces wrinkling and seat-sag in skirts and trousers.
• Creates a base for boning channels in fitted bodices. See Boning a Bodice.

Choosing an underlining fabric

Match the underlining to the effect you want, and choose a fabric no heavier than the fashion fabric unless you want stiffness:

• Silk organza: crisp, light and almost weightless; adds body without bulk. The classic choice for silk and couture work.
• Cotton batiste or lawn: soft and breathable; adds opacity with little change in drape.
• Cotton muslin/calico or broadcloth: firmer support for structured dresses and bodices.
• Coutil or cotton canvas: strong support for boned bodices and corsetry.
• Flannel (cotton or wool) or lightweight wool: adds warmth to coats and jackets.

Pre-wash or pre-shrink both fabrics the same way, so they don't shrink at different rates later.

How to attach it

1. Cut the underlining from the same pattern pieces, on the same grain as the fashion fabric. Transfer darts and markings to the underlining.
2. Lay the fashion fabric wrong side up, place the underlining on top with markings facing you, and smooth the two together.
3. Allow for turn of cloth on curved pieces. For pieces that wrap around the body (sleeves, skirts), lay the two layers over a curved surface such as a tailor's ham or a rolled towel while pinning, so the outer fabric isn't pulled tight over the underlining when worn. Trim any excess underlining at the edges.
4. Baste the layers together by hand around all the edges, just inside the seamline, and along the centre of darts. Hand basting with silk thread holds well without marking. Some sewists machine baste, which is quicker but can shift slippery fabrics.
5. Sew the garment as normal, treating each underlined piece as a single layer. Remove the basting once seams and darts are sewn.

"Flatlining" is often used as another word for underlining. It's also the name of a variation in which the underlining is cut with wider seam allowances, which are wrapped around the fashion fabric's raw edges and stitched down, finishing the edges at the same time. That version suits unlined garments where the inside will show.