The Post Comments widget is a Theme Element. It is one of the available Single Post Template widgets that is used to dynamically display a post』s user comments.
Content
Comments
Skin: Select an available skin. Currently one skin is available, the Theme Comments skin. The Theme Comments skin uses the currently active theme comments design and layout to display the comment form and commentsSource: Select the source of the user comments, either those from the Current Post or Custom. If Custom is chosen, the following option becomes available: Search & Select: When you start typing a specific name of a post, you will get the autocomplete full name of the post, which you can then select. The comments from the selected post will be displayed.
Note: The Post Comments widget will only work if comments are allowed. This setting is controlled via WordPress Discussion settings.
Read the tutorial showing how to Create a Single Post Template