An expansion's //first set// is simply the set of all tokens that can begin the expansion.

The most trivial example imaginable is any expansion that begins with a single token. Thus, the first set of:

<code>
    "foo" "bar" "baz"
</code>

is simply a set with one element, "foo".

The first set of 

<code>
    ("foo" | "bar") "baz"
</code>

has two elements, "foo" and "bar". The above expansion must necessarily begin with one of those two tokens.

For example, if the production ''Foobar'' is defined as:

<code>
    Foobar : "foo" | "bar";
</code>

then the //first set// of the expansion: 
   
<code>
     [ Foobar ] "baz"
</code>

contains the three tokens, "foo", "bar" , and "baz. To say that a grammar is LL(1) is equivalent to saying that, at any [[choice points | choice point]] the expansion's //first set// is enough to decide whether or not to enter the expansion.