Underscores are commonly used in variable names in many programming environments. However, since the underscore has a special meaning in Mathematica, it is not allowed in Mathematica identifiers. While digits are allowed, you may not use a digit as the first character in a variable name. Note also that in Mathematica predefined functions and constants always have names that begin with capital letters. It is therefore recommended that you choose names starting with lowercase letters. This way you'll never create a conflict with a built-in object.