Verbs can have either a single argument (to the right of the verb) or two arguments (to the left and right). A verb with only a right argument is referred to as a monad, a verb with both right and left arguments is referred to as a dyad.
Often the same name or primitive will have different meanings depending on whether it is called with one or two arguments. In that situation we talk about the monadic form of the verb, or the dyadic form of the verb.
For example the primitive verb % will return the reciprocal of its right argument when called monadically, but when called dyadically it will divide the left argument by the right argument.
% 4 NB. monadic % 0.25 4 % 2 NB. dyadic % 2
In the world of J, the symbol y is often used to refer to a verb's right argument, while x is commonly used to refer to a verb's left argument.
% y NB. monadic form of % x % y NB. dyadic form of %
Other languages, such as J's ancestor APL, also allow a function to be executed with no arguments (niladic form). J has no niladic case. If you execute a verb in the session without any arguments it will display its definition.
Verbs that don't require or ignore their arguments are often executed by giving them an empty right argument. For example the verb 6: ignores its arguments and returns the number 6
6: '' NB. execute 6: with empty right argument 6 6: NB. "execute" verb with no arguments: displays verb definition 6:
Often we want to call a verb with more than 2 arguments. There are a number of Strategies for dealing with Multiple Verb Arguments.