I decided to underline my garment when I purchased the fabric. Underlining in some cases provides stability. But underlining also can play other roles in a garment depending on the effect you want to achieve. You have to consider what you want the underlining area to do: drape, stand up, or fold. For a fitted, boned bodice your may use flannel to camouflage and muslin for strength. Silk organza would be perfect to add lightweight support in a skirt or sheath dress.
For my Summer Dress, I decided to use silk organza because it's stable and easy to work with. It's a perfect match for silk dupioni. In any case, you want to cast the correct underlining. Here are some options to consider:
Lofty - single or double faced cotton flannel camouflages construction details.
Heavy - Cotton broadcloth, muslin, crinoline, and twill offer firm support.
Drapey: Silk crepe de chine and rayon maintain movement.
Light - Silk chiffon is gentle; silk organza, crisp. Cotton voile or cotton batiste add soft support.
Pretreat your fabrics. Steam the fashion fabric and wash any cotton underlining in hot water. Do not wash silk organza; it won't shrink.
Fabric and Underlining Combinations -
Silk Chiffon and Silk Chiffon Underlining
Linen-Silk Blend and Muslin Underlining
Four-Ply Silk Crepe and Silk Crepe De Chine Underlining
Silk Dupioni and Silk Organza Underlining
Wool Crepe and Silk Organza Underlining
That it for the installment. Next Marking the inner layer and Aligning the underlining and fabric.