Every totally ordered set 
is associated with a so-called order type. Two sets
and 
are said to have the same order type iff
they are order isomorphic (Ciesielski 1997, p. 38;
Dauben 1990, pp. 184 and 199; Moore 1982, p. 52; Suppes 1972, pp. 127-129).
Thus, an order type categorizes totally ordered
sets in the same way that a cardinal number
categorizes sets. The term is due to Georg Cantor, and the definition works equally
well on partially ordered sets.
The order type of the negative integers is called 
(Moore 1982, p. 62), although Suppes (1972, p. 128)
calls it 
.
The order type of the rationals is called 
(Dauben 1990, p. 152; Moore 1982, p. 115; Suppes
1972, p. 128). Some sources call the order type of the reals 
(Dauben 1990, p. 152), while others call it 
(Suppes 1972, p. 128).
In general, if 
is any order type, then 
is the same type ordered backwards (Dauben 1990, p. 153).
