2. Undergarments - The essential Regency undergarment was the chemise, also known as the "shift," a sleeveless, mid-calf length slip of white cotton or muslin. Beneath it one only wore a pair of stockings, knee-high, in silk or wool, held up by ribbon garters. Over it, one wore stays, the Regency version of a corset. Full-figured women might choose hip-length stays to shape the torso, but usually "stays" merely hugged the ribcage to support the bosom. Stays were made of a sturdy canvas cloth, often faced with silk, reinforced inside with whalebone, which had a bit of give so they were not terribly uncomfortable compared to the corsets of the preceding early Georgian and following Victorian periods. They typically laced up the back.
Finally, the lady donned one or more petticoats over her chemise, tied at the waist with "tapes" in the front or back. This was for warmth, modesty, and to help give shape to her gown. Petticoat length matched the length of the gown she would be wearing--floor length in most cases, but shorter for walking dresses.

3. Casual or "Undress" - Also called "deshabille." The Regency lady spent a good deal of her day changing clothes, each outfit growing more formal as the day wore on. The first outfit of the day was "morning dress" and would typically be worn till about
noon--loose, casual, inexpensive fabrics, no fuss. Worn for breakfast, getting her day organized with the housekeeper and/or her personal secretary, and receiving visits from close female friends, who, in turn, would be wearing "visiting gowns." Still informal, but a slight bit more dressy since the friend was the one who had ventured out in public.
Walking dresses fell into the undress category. If the lady opted to go out for a "brisk constitutional" she donned a walking dress. The English were two hundred years ahead of the current craze for walking, the perfect low-impact aerobic exercise. Walking was encouraged for health even during the Regency. The skirts of the walking dress were shorter and the whole dress was cut to allow greater freedom of movement.

4. "Half-Dress" - Mid-level formality worn from the early afternoon to early evening. Some examples, going from less to more formal: Promenade Gown--could be casual or half dress, for strolling in the park at the fashionable hour (5:00 p.m.), paying afternoon calls on acquaintances, or shopping in the fancy shops of Bond Street. Carriage Dress--a bit fancier than Promenade, worn while driving in the park showing off one's "equipage" (carriage).
Dinner Gown and Opera Gown--these sound like they would be extremely formal, but they're still not as elaborate as "full dress." A lady dressed for dinner even when dining at home. The gown would be elegant, but not overly rich. Going to the opera in the Regency was not the ultra-formal affair it had been in earlier years and became again later in the Victorian. What determined formality was the length of the skirts (no train), the richness of the materials used, and the amount of decoration on the gowns. Accessories (hats, jewels) could help fine-tune the level of formality the woman wanted to achieve with a particular outfit.