Why are the Fed's goals often described as the "dual mandate" and not the "triple mandate"? The Federal Reserve Act Section 2A states:


  The Board of Governors of the Federal Reserve System and the Federal Open Market Committee shall maintain long run growth of the monetary and credit aggregates commensurate with the economy's long run potential to increase production, so as to promote effectively the goals of maximum employment, stable prices, and moderate long-term interest rates.


Three goals are given, namely:


Maximum employment;
Stable prices; and
Moderate long-term interest rates.


So why is it that many (or even most) writings* usually refer only to the dual mandate, i.e. the first two goals of maximum employment and stable prices? Why do they seem to ignore the third goal of moderate long-term interest rates?

Is it because the third goal is less important? Or perhaps because the third goal somehow follows automatically if the first two are fulfilled?

*Examples: Chicago Fed, Richmond Fed, St. Louis Fed. Indeed, the last says the following but gives no explanation:


  But wait a minute: None of this sounds like a dual mandate. Rather, it sounds as if Congress has given the Fed three mandates—if not four. Over the years, the mandates were boiled down to two: maximum employment and price stability. And now, some say the central bank can focus on price stability.
The textbook answer is that the interest-rate objective is considered redundant (implied by) price stability, in the long run, i.e. low inflation is expected to bring down interest rates anyway.

It's interesting that Congress actually added the (3rd) interest rate objective in 1975-1977. Maybe there's some historical context that explains their insistence on it. I don't know more.



As for "some say the central bank can focus on price stability" that's an allusion to ECB's primary mandate (sometimes/unofficially called "single mandate"), no doubt.



And hat-tip to BKay for bringing this up, there are indeed more similarities between the Fed and ECB; even though not by law...


  As
  explained in the 9th edition of the Federal Reserve’s Purposes and Functions, published in
  2005: “Stable prices in the long run are a precondition for maximum sustainable output
  growth and employment as well as moderate long-term interest rates.” In 2004, Chairman
  Greenspan pointed to the primacy of price stability in explaining the success of Federal
  Reserve policy since 1979. As he explained, this resulted from “maximizing the probabilities
  of achieving our goals of price stability and the maximum sustainable growth that we
  associate with it” (Greenspan, 2004).
  With this interpretation of the Federal Reserve’s mandate, closer comparison with that of
  the ECB suggests more similarities than differences. This is because while the mandate of
  the ECB explicitly recognizes the primacy of price stability, it does not ignore other
  objectives. 


Furthermore, analysis of actual macro models used by the ECB and the Fed (as they were in 2008 anyway) points to a number of "real" similarities, including the fact that neither is strictly IT (Inflation Targeting) in the sense of NCM (New Consensus Macroeconomics)... unlike that of the Bank of England, which was found to be IT.