A variable is a piece of data with a varying value. Within a procedure, each variable is known by a unique name and is always referred to by that name.
When you choose a name for a variable, the first character must be one of:
A B C...Z ! ? _
Lowercase letters are also allowed as a first letter. The interpreter changes them to uppercase.
The rest of the characters can be any of the preceding characters and also 0 through 9.