Default

Default[f]
gives the default value for an omitted paramter of f.
Default[fk]
gives the default value for a parameter on the kth position.
Default[fkn]
gives the default value for the kth parameter out of n.

Assign values to Default to specify default values.

Default values are stored in DefaultValues:

You can use patterns for k and n:

Note that the position of a parameter is relative to the pattern, not the matching expression: