Treasury Securities ("T-Secs", also known as TCM, or CMT, or T-Sec) values are calculated by the Treasury Department and reported by the Federal Reserve in Publication H.15. Below, you will find current and historical monthly yields for 3 month, 6 month Treasuries, as well as values for 1-, 2-, 3-, 5-, 7-, 10-, 20-, and 30 year treasuries.
The official name of this index is "Monthly Average Yield on U.S. Treasury Securities Adjusted to a Constant Maturity of (number) Years" (or "six months", or "2 years", etc.). Confusion can arise when lenders use the term "One Year Treasury Bill"; the 52-week bill is a totally different index, and rarely used on ARMs.
Treasuries have both monthly and weekly values; for ARMs, the weekly value is most often used. Monthly values are rarely employed, excepting where a Moving Treasury Average (MTA, aka 12-Mat) is used. If you think your ARM is tied to a monthly value, you'll want to check your check your mortgage Note or Adjustable Rate Rider for specific details.
However, monthly values are excellent for tracking changes in interest rates and make great companion series for many other economic indicators. Need weekly treasury index values instead?