Define 0 to be the default argument for a function f:

_. represents an argument that can be omitted:

If the second argument is omitted, the default value of 0 is used instead:

If both arguments are omitted, both are taken to have the default value 0:

Define default values for the first and second arguments:

If an argument is omitted, the corresponding default value is used:

Define 2 to be the default argument for the pattern f[_.,_]:

Define 3 to be the default argument for the pattern f[_.,_,_]:

With a two-argument pattern, the default value of 2 is used for the omitted argument:

With a three-argument pattern, the default value of 3 is used for the omitted argument instead:

Assigning a value to Default[f] will define default arguments for all positions:

Assigning a value to Default[f] only affects pattern matching for f:

It does not on its own create any transformation rules for f[…]:

Use a pattern involving Optional in the definition of f to create a default evaluation:

Defaults can be defined by immediate or delayed assignments:

Use ClearAll to clear default values:

Clear does not clear them: