This post offers a plain vanilla explanation of the debt limit. This is basic background aimed at fiscal policy novices. I oversimplify in a few places for ease of understanding and push some caveats and complexities down to footnotes. Some of my past readers may find this too elementary, but it’s a foundation I can build upon in future posts that will offer more detail and discuss the current stalemate.
The federal government’s fiscal year begins October 1, so federal fiscal year (FY) 2013 just ended and FY 2014 just began last week. In FY 2013 the federal government collected about $2.813 trillion in revenues and spent about $3.455 trillion. The difference between those two numbers, $642 billion, is the unified budget deficit for Fiscal Year 2013. We say unified budget deficit because we’re looking at all money flowing into the federal government (revenues) and all money flowing out (spending), and not distinguishing among different sources or uses of those funds. In other contexts, usually having to do with Social Security, we might want to treat on-budget and off-budget spending and taxes differently, but for this discussion we will use unified numbers.
I think of these enormous numbers as representing flows of cash. (Please see the footnotes below for two caveats:  ) The key question: if last fiscal year the government spent $3.455 trillion in cash but collected “only” $2.813 trillion in cash, where did it get the other $642 billion in cash it needed to pay the rest of its bills?
The federal government borrowed it. The Treasury department sold IOUs that we call Treasury bonds (technically bills, notes, and bonds, depending on the timeframe). Investors paid cash for these IOUs, and in exchange they got a promise that Treasury will pay them back later, in full, with interest, and on time. We say that Treasury issued debt to raise cash. Treasury’s issuance of debt is a means to an end: it is the mechanism Treasury uses to get the cash it needs to pay all the government’s bills (which we call government obligations) on time.