Pickling Onions

I recently came across a recipe for pickling onions. I have never heard of them and could not find them in the grocery store. What is the difference between pickling onions and regular onions? —R.R., Keene, Ohio

Pickling onions, sometimes called "boilers," are small tender white or yellow onions with a mild flavor. They range in size from 1/2 inch to 1 inch in diameter. Their small size and mild flavor sets them apart from the typical yellow onion. Pickling onions can also be used in place of pearl onions or any small mild-flavored onion called for in recipes. Pickling onions can often be found during summer and fall in the produce department of grocery stores.