Question about first-child JQuery selector

[Hussein Baghdadi]

Hey,
I'm reading the second chapter of "JQuery In Action"
To get languages names, this selector is used:

table#languages tbody td:first-child

tbody contains four <tr> and each <tr> contains three <td>
Why the previous selector get the first <td> of each <tr>?
Thanks.

[Eric Pascarello]

Not sure if this answers your question:

:first matches only a single element
:first-child matches an element for each of its parents

Eric