The word "betweenbrain" uses 12 letters: abbeeeinnrtw.
betweenbrain is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for betweenbrain in these online resources (some words may not be found):
Wiktionary - OneLook Dictionaries - Merriam-Webster - Google Search
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Words formed by adding one letter before or after betweenbrain (in bold), or to abbeeeinnrtw in any order:
s - betweenbrains