Callable Certificate of Deposit Overview

Callable CDs are common for long-term brokered CDs, but they're not too common for CDs you get directly from banks. PNC Bank has recently been offering callable CDs directly, and this got the attention of Jim at Bargaineering. He has a useful post with an overview of callable CDs. How much higher does the rate of a callable CD have to be compared to a regular CD for it to be a good deal?