Macroeconomics - IS/LM model

Economist and columnist Paul Krugman advises us to think in terms of models.  One of the simple and important models for Keynesian economists is the IS-LM model.  CIP began a discussion of it.  Fortunately, it is simple enough to understand, there is an excellent exposition here.  The four youtube videos take just under a half-hour to watch, but after that you are equipped to think about the economy in terms of this simple model.