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What's the difference between these selectors?

:nth-child :first-of-type :only-of-type

I watched the video explaining these, but I don't get the difference very much. Could someone explain?

2 Answers

Nick Pettit
Nick Pettit
Treehouse Teacher

Hi Alfons,

  • :nth-child takes an integer value that allows you to select a child element. For example, if you have a <ul> with list items and you want to only select the 3rd item, you can do it with :nth-child instead of applying a class or ID to the 3rd item. Here's an example on CodePen that shows this in action: http://codepen.io/nickpettit/pen/FnejA
  • :first-of-type is a little tricky to explain in words, but it matches the first element that is of the type described in the selector. For example, if there were several nested <section> elements that also had siblings of other types, and you just wanted to select the first <section> (and not the first element) then you want :first-of-type. Here's an example on CodePen: http://codepen.io/nickpettit/pen/wiGrq
  • :only-of-type is similar to the previous example, except it will select elements only if they have no other similar siblings. Here's the same example in CodePen: http://codepen.io/nickpettit/pen/xKHdf - In this example, try changing header to section, and you'll see that it doesn't select the section elements. That's because there are multiples. However, there's only one header.

I hope that helps!

Amazing, thanks Nick! I now understand these concepts.

I am impressed that the staff and mods of Treehouse will actually answer and participate on the forums. This is truly a great site.

Nick Pettit
Nick Pettit
Treehouse Teacher

No problem, happy to help! :)

Nick Pettit
Nick Pettit
Treehouse Teacher

Weird, thanks James! Fixed it.

James Barnett
James Barnett
39,199 Points

TL;DR - Any time you are confused on what a CSS selector does you can always use the SelectORacle, which is where I got the wording for the explanations here.

First some background ...

Parent & children in CSS:

Siblings in CSS:

Descendants in CSS:

Now that the background is covered here are some examples:

div:nth-child(2) p

Selects any p element that is a descendant of a div element that is a second child.

div p:first-of-type

Selects any p element that is a first child of its type that is a descendant of a div element.


Selects any span element that has no siblings of its type.

Here's a quick demo to see how this would work.

Images shamelessly lifted from the most excellent Selectutorial

That was an excellent explanation as well. Thanks!