The Merriam-Webster dictionaries are recommended by Spelling Power author Beverly L. Adams-Gordon. The four volumes are graded for age appropriateness by the number of words listed, yet they all maintain the same diacritical marking system. In addition to those listed above, there are other advantages to these dictionaries. Because the company primarily produces dictionaries and thesauruses, you can count on high quality. Also, they are the original "Webster's" publishing company, now known as Merriam-Webster. Not every "Webster" dictionary is printed by the original company. Since the name is not trademarked or copyrighted, anyone can use it. Make sure you buy a "Merriam-Webster" when you look for dictionaries.