Although the Oxford English Dictionary defines more than 750,000 English words, there are only about 860 words that have their letters arranged in alphabetical order. These are known as abecedarian words (from the Latin word, acededarius, meaning “alphabetical”).
Excluding the word “a”, the shortest words with letters in alphabetical order, containing only 2 letters, are: am, an, be, is, it, no.
The word ace contains 3 letters in alphabetical order.
There are many words with their letters in alphabetical order that contain 6 letters: abhors, accent, access, almost, biopsy, billow, chintz, effort.
The longest word with letters in alphabetical order is Aegilops (8 letters). Technically, this is a Latin term for North American and Eurasian plants in the grass family, Poaceae (generally known as goatgrasses), that are the wild ancestor of modern domestic wheat. In medicine, an aegilops is an abscess or ulcer in the outer or inner corner of the eye. However, the longest English words with letters in alphabetical order, containing 7 letters are: beefily, billowy.
Number with letters in alphabetical order: forty
Number with letters in reverse alphabetical order: one
Longest word with letters in reverse alphabetical order: punctoschmidtella (17) and spoon-feed (9 letters)
Shortest word with all vowels in alphabetical order: aerious (7 letters)
Longest word with all vowels in alphabetical order: phragelliorhynchus (18 letters), adventitious (12), abstemious (10), facetious (9)
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Read related posts: What is the Longest Word in English Language?
Word Oddities: Fun with Vowels
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For further reading: Wordplay: A Curious Dictionary of Language Oddities by Chris Cole, Sterling (1999). rinkworks.com/words/oddities.shtml
listverse.com/2007/12/03/25-english-language-oddities/
http://www.oxforddictionaries.com/us/definition/american_english/aegilops
http://math.cmu.edu/~bkell/alpha-order/
Atkins Bookshelf 
The word “pancreaticoduodenostomy” doesn’t have its vowels in alphabetical order, as the letter “a” appears after an “e,” an “o” appears after a “u,” and an “e” appears after an “o” and a “u.” I think the code that you ran to find these words didn’t check that none of the vowels already checked for didn’t appear again in the word after a different vowel. Also, the word “am” (the first person singular conjugation of “to be”) should be included in the list of 2-letter alphabetical words.
Hi Nate: Thanks so much for catching those. Have updated post.
“The shortest word with letters in alphabetical order is ace (only 3 letters).”
No. No is shorter. With two (consecutive) letters.
An is an alternative with two letters. So it is. (So it be)
All? All is another abecedarian with three letters. Another hit.
A is the first and shortest.
Will: Thanks so much for your contributions. Very much appreciated. Have updated the post.
Do you have the full 860 words?
Good question. Unfortunately, no. However, with a bit of sleuthing on the internet you can find them all. If you do happen to find them all (ideally in one place), please let me know. Cheers. Alex
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