The Treasury Security (or "T-Sec") indexes are calculated by the Treasury Department and reported by the Federal Reserve; these indexes have both weekly and monthly values. Most lenders use the weekly value on their ARMs. These indexes reflect the state of the economy, and respond quickly to economic changes.
The most commonly used is the One Year Treasury Security; it's used on ARMs with annual rate adjustments. Confusion can arise when some lenders use the term "One Year Treasury Bill." Most one-year ARMs are tied to the Constant Maturity of the One Year Treasury Security.