What: Shares of National Penn Bancshares (NASDAQ: NPBC) traded higher by about 16% at the market open today after announcing that it would be acquired by BB&T Corporation (NYSE:TFC).

So what: BB&T Corporation has agreed to pay $13 per share for National Penn in a cash and stock transaction. National Penn shareholders can elect to receive 0.3206 shares of BB&T or $13.00 in cash for each National Penn share they own. The deal values National Penn at 2.2 times tangible book value.

National Penn Bancshares was teetering on the $10 billion in assets line, the point at which a bank is subject to a new regulator, a cap on debit card interchange fees, and additional stress testing. On the most recent conference call, company executives discussed that it wouldn't cross the line with organic growth, only by acquiring another bank "of size" to take it over that threshold.

As it turns out, National Penn is a better acquisition target than an acquirer. BB&T hopes that it can achieve annual cost savings of $65 million by slashing 30% of National Penn's non-interest expenses.

Now what: National Penn Bancshares will help BB&T grow its business in the Mid-Atlantic, and more specifically, in Pennsylvania. Last year, BB&T Corporation acquired Susquehanna Bank, a Pennsylvania bank with 260 branches. National Penn Bancshares' 124 offices and branches will make BB&T Corporation the fourth-largest bank in Pennsylvania by deposits.