C# provides various number of keywords to decide the scope, those are

• private(default one),
• public,
• internal,
• protected and
• protected internal.

If you create any type without these keywords, by default it will take as private.

By default, for all members of a class the visibility is private. Default visibility for a class is internal.

Read more detailed article on "Access Specifiers or Access Modifiers in C#" here.