What jQuery version to mention in code

Question

I'm using jQuery, and I'm little confused about what version is included at start of my page. From my experience with other software I understand that each new version contains most old features, and offers some new ones. If latests jQuery version is 1.9 for example, why would I mention this:

<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="../jqm145/jquery.mobile-1.4.5.min.css">
<script src="../jq1-11-3/jquery-1.11.3.js"></script>
<script src="../jqm145/jquery.mobile-1.4.5.min.js"></script>

How do I check if a function will work in a specific version of Jquery?

Furthermore, the mobile JQuery version 1.4.5 and "plain" JQuery is 1.11.3. Why is this the case?

Answer 1

jQuery mobile (perhaps confusingly) is not a mobile version of jQuery, it's a library based on jQuery that contains controls, logic, etc. for Mobile devices.

jQuery has remained backwards compatible until version 3.0.0, in which support for older browsers has been dropped.

As a rule of thumb, it's good to update both libraries at the same time if one depends upon the other.