Containers are considered 'wrapper's' of modules within a DotNetNuke page.

Your module has many different types of containers to choose from. To select a container, hover over the pencil Icon of the module that you want to wish to alter

Inside the module settings select the 'Page Settings' tab, click the 'Module container:' drop down and choose the container that you want to use.

By selecting the preview button you can view what the container will look like before you apply it.

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Once you have the container that you wish to apply to the module select the 'Update button' to apply the changes,