 What are the rules for a valid variable name in C? A valid variable name must begin with a letter or underscore followed by letters, digits, or underscores. That means that these variable names are valid, but these aren't. There are also some key words in the C language known as reserved words that you can't use as variable names. Here are most of them. The link in the video description has a complete list. A bit more about valid variable names. While you can start variable names with an underscore, you probably shouldn't. By convention names beginning with underscores are used for system level identifiers. Also, the definition of a letter includes unicode alphabetic characters, not just the letters A through Z. That means that this is a valid C program, but please use English for your variable names when you do assignments for this course.