Rolling one-period bond account
= Rolling one-period bond account
{title2=$\beta_t=\beta_{t-1}/P_{t-1}(t)$}
Reinvest each maturity payment in the next unit-principal <zero-coupon bond>. Positive bond prices make the resulting <self-financing portfolio> strictly positive, so it can be used as a <numéraire>. Its one-step growth is the reciprocal of the current one-period bond price.