Sophie Théallet has been making shirtdresses since 2005, the year she opened her own house after ten years at Azzedine Alaïa's side. The shirtdress is the garment she has never stopped working on. This is what she looks for when one fits, and how to choose your size at Room 502.

What should a shirtdress feel like at the shoulder and collar?

Like a good shirt. The shoulder seam sits on the edge of your shoulder, not down the arm. The collar closes without pulling, and the buttons lie flat across the bust, with no gaps between them. If the placket gapes when you sit or reach, take the next size up.

Where should the waist sit?

At your natural waist. Our shirtdresses are cut close to the body through the bodice and define the waist. The Zoe has a fitted waist and a full pleated skirt. The Stephanie and the Charlotte have a built-in belt that adjusts at the front, so you fasten it exactly where your waist is. The waist should mark your shape without pinching. If it does, size up.

How long should a shirtdress be?

It depends on the dress. The Zoe is a midi: at 5'4" and under it may sit long, and an easy hem takes care of it. The Stephanie and the Charlotte sit at the knee. Our model is 5'10" and wears a size 4/36.

Does the fabric change the fit?

Yes. The Zoe is 100% Imperial cotton poplin: crisp, structured, with no stretch, so it holds its line and gives nothing at the waist. The Black Stephanie is cotton with a touch of stretch (97% cotton, 3% elastane) for more ease when you move. The Charlotte is viscose, which drapes and moves more softly.

Should I size up or down?

Our cut is tailored, close to the body the way a fine shirt is. If you like a little more ease, size up one. Between two sizes, take the larger, or email us your measurements at info@room502.com and we will guide you before you order. Sizes run 2/34 to 14/46, US and French. The full chart is on our Size & Fit page.

Do shirtdresses have pockets?

Ours do. The Zoe has hidden side pockets, and the Stephanie has front pockets.

How do I wear it from day to evening?

By day, roll the sleeves and leave the top buttons open, with flats or sneakers. For evening, button it all the way up and add heels.

What if it doesn't fit?

Try it at home. Shipping to the US and Canada is free on every order, and returns are free on orders of $450 or more. Keep it only if it's right.

Sophie Théallet is a CFDA/Vogue Fashion Fund winner. Her shirtdresses are cut in limited runs and sold only at room502.com. See all shirtdresses.