Which herb is the main flavoring in the French sauce béarnaise?

The main flavoring herb in French béarnaise sauce is tarragon.

Béarnaise is a warm emulsion made from clarified butter, egg yolks, and a reduction of white-wine vinegar, shallots, and pepper. Tarragon supplies its distinctive anise-like aroma and is the flavor that most clearly separates béarnaise from its parent sauce, hollandaise.

The sauce is considered a child of hollandaise and is traditionally served with steak and other meats. Its name suggests an origin in Béarn, a region of southwestern France, although the sauce is commonly associated with a restaurant at Saint-Germain-en-Laye near Paris. A popular account credits chef Jean-Louis-François Collinet with creating it in the nineteenth century.

Chervil may also appear in traditional recipes, along with shallots and peppercorns, so it is a frequent source of confusion. Chervil contributes a delicate parsley-like note, but tarragon remains the canonical answer to the question about béarnaise's main flavoring.