Function relating yield to maturity, the basis for extracting discount factors and forward rates. Three economic shapes: normal (upward — positive term premium), inverted (downward — recession signal, ten-out-of-ten US recessions since 1955 preceded by 2s10s inversion), and flat. Decomposable into level, slope, and curvature factors (PCA), which together explain ≥ 95% of variance. Built today via multi-curve bootstrapping (OIS/SOFR discount + per-tenor forecasting curves); the choice of interpolation (linear-on-rates, cubic-spline, monotone-convex) materially affects implied forward rates.