Cos lettuce is a round-hearted variety renowned for its smooth, firm texture and full-bodied flavour. In the USA and France it is known as Romaine.
There are several types of cos lettuce, and one of the most popular today is Little Gem, which looks like a small, tightly-packed version of the regular cos you'll find on sale.
When buying, ensure the lettuce is a vibrant green colour and that the leaves are standing proud, not droopy and yellowing.
Cos is a good salad base, whether served alone or combined with other leaves. It is essential for true Caesar salad. Thanks to the firm texture of the leaves, it makes a good wrapper for soft savoury mixtures. It can also be cooked (briefly) as a vegetable, and made into soup.