Take a unit-face zero-coupon bond with . Its continuously compounded zero-coupon yield to maturity, instantaneous forward rate, and short rate are
The last equality assumes the appropriate maturity differentiability at the diagonal. Integration of the forward-rate definition and differentiation of the yield definition give
At fixed observation time , the yield curve plots this yield against maturity ; it describes today's discount prices at different horizons. Forward rates are extracted from these prices, not identified with the subsequent realized path of the short rate.