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Marking a lecture as free preview lets potential students sample the course content without spending money. This feature does not require students to be signed in to the website to preview sample lectures. A lecture can be enabled for a free preview after publishing the lecture and setting the lecture for "Free Preview" in course materials page.
In the course landing page, you can see "Preview Course" button. You need to set at least one lecture for 'free preview' to enable this button.
When a student try to access other lectures in the course, it will show the lecture in locked status.