Negative Sign and Infinity  _ 
The symbol _ followed by a digit denotes a negative number
(as in _3.4), and denotes infinity when used alone,
or negative infinity (in __). It is also used in names,
as discussed in
Part I and in
Part II Section I.

2 % 0 NB. Two divided by zero _ 10 ^. 0 NB. Base ten logarithm of zero __ _2 _ 3 + 5 3 _ 8 integer_test=: =<. NB. Use of break in name integer_test 3 3.5 1 0Although 2 may sometimes be used instead of _2, it is important to understand that the former is the application of a function to the number 2, whereas the symbol _ is an indivisible part of the number representation, just as the period is an indivisible part of a number such as 8.9 .