This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.
M, N, B, D, P ?
series/series.01.s
T. If you say the sounds these letters make out loud, you will see
that the next letter is T. The letters are in the order of where the
sounds are formed in the mouth, from back to front.
 
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