Definition:Order Type of Rational Numbers/Historical Note

Historical Note on Order Type of Rational Numbers
The use of $\eta$ to denote the order type of $\struct {\Q, \le}$ dates back to the writings of and.

According to and  in their  of $1996$, this order type, along with that for $\struct {\R, \le}$, has not been greatly studied in recent years.

$\eta$ is in fact far more often used to denote a general ordinal.