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