This is a little bit of a myth, but also a little bit true. Virtually all plant-sources of protein have all essential amino acids. It’s just that legumes have a slightly lower amount of one amino acid compared to what’s in our muscles, and grains have a slightly lower amount of another. So while they do complement each other and ensure plenty of all amino acids, it’s also actually not that hard to get all necessary protein from one or the other just by eating a bit more of it.

In other words, if you go a day or even a few days only having grains or only having legumes, it’s probably not going to hurt you, so there’s no need to be overly concerned about protein combining. But in any case there are other reasons why it is better to eat a variety of foods, so getting grains and legumes is still better.