There is a special identifier that one can use in a formula to mean all the variables, it is the `.`

identifier.

```
y <- c(1,4,6)
d <- data.frame(y = y, x1 = c(4,-1,3), x2 = c(3,9,8), x3 = c(4,-4,-2))
mod <- lm(y ~ ., data = d)
```

You can also do things like this, to use all variables but one (in this case x3 is excluded):

```
mod <- lm(y ~ . - x3, data = d)
```

Technically, `.`

means *all variables not already mentioned in the formula*. For example

```
lm(y ~ x1 * x2 + ., data = d)
```

where `.`

would only reference `x3`

as `x1`

and `x2`

are already in the formula.