## $MU

`$MU` is the head of dummy indices which may be introduced by `Chisholm`, `Contract`, `DiracSimplify`, `FermionSpinSum` and various QCD functions. By default it is unset, but can be set to anything.

### See also

[Overview](Extra/FeynCalc.md), [Chisholm](Chisholm.md), [Contract](Contract.md), [DiracSimplify](DiracSimplify.md).

### Examples

```mathematica
$MU
```

$$\text{\$MU}$$