As an author, blogger and copy editor, I have a close and continuing interaction with Merriam-Webster's Collegiate Dictionary, Eleventh Edition (Web11), as my US spelling guide and The Chicago Manual of Style, Sixteenth Edition (16CMS), as my US grammar reference.
Should 16CMS and Web11 ever contradict, 16CMS rules—particularly 16CMS 7.85 which overrides some Web11 spellings. Also Microsoft Word (MSWord) does not strictly follow Web11 (except for maybe its 2013 update, which I understand allows you to select Web11 as your default dictionary). Just because MSWord's spell-check program accepts a word as valid doesn't make it necessarily so.
Note that the first Web11 entry is the preferred spelling.
Our English language is full of exceptions and surprises, even when comparing the UK standard to our usage in the States. Here are just a few of the words I find most often misspelled:
toward [Web11's preferred spelling is without a terminal S]
seat belt (n)
rib cage (n)
Denise Barker, Author, Blogger, Copy Editor
Books that Build Character(s)
When you give someone a book, you don’t give him just paper, ink, and glue. You give him the possibility of a whole new life. Christopher Morley
The best inheritance you can leave your kids is an example of how to live a full and meaningful life. Dan Zadra