The exact rules of a naming convention depend on the context in which they are employed. Nevertheless, there are several common elements that influence most if not all naming conventions in common use today.
Letter case and numerals
Some naming conventions limit whether letters may appear in uppercase or lowercase. Other conventions do not restrict letter case, but attach a well-defined interpretation based on letter case. Some naming conventions specify whether alphabetic, numeric, or alphanumeric characters may be used, and if so, in what sequence.
Multiple-word identifiers
A common recommendation is "Use meaningful identifiers." A single word may not be as meaningful, or specific, as multiple words. Consequently, some naming conventions specify rules for the treatment of "compound" identifiers containing more than one word.
As most programming languages do not allow whitespace in identifiers, a method of delimiting each word is needed to make it easier for subsequent readers to interpret which characters belong to which word. Historically some early languages, notably FORTRAN (1955) and ALGOL (1958), allowed spaces within identifiers, determining the end of identifiers by context. This was abandoned in later languages due to the difficulty of tokenization. It is possible to write names by simply concatenating words, and this is sometimes used, as in mypackage for Java package names,[2] though legibility suffers for longer terms, so usually some form of separation is used.
Delimiter-separated words
One approach is to delimit separate words with a non-alphanumeric character. The two characters commonly used for this purpose are the hyphen ("-") and the underscore ("_"); e.g., the two-word name "two words" would be represented as "two-words" or "two_words".
The hyphen is commonly used by when writing COBOL (1959), Forth (1970), and Lisp (1958); it is also common in Unix for commands and packages, and is used in CSS.[3]
By contrast, languages in the FORTRAN/ALGOL tradition, notably languages in the C and Pascal families, used the hyphen for the subtraction infix operator, and did not wish to require spaces around it (as free-form languages), preventing its use in identifiers.
An alternative is to use underscores; this is common in the C family (including Python), with lowercase words, being found for example in The C Programming Language (1978),[4] and has come to be known as snake case or snail case. Underscores with uppercase, as in UPPER_CASE, are commonly used for C preprocessor macros, hence known as MACRO_CASE, and for environment variables in Unix, such as BASH_VERSION in bash. Sometimes this is humorously referred to as SCREAMING_SNAKE_CASE (alternatively SCREAMING_SNAIL_CASE).
Letter case-separated words
Another approach is to indicate word boundaries using medial capitalization, called "camelCase", "PascalCase", and many other names, thus respectively rendering "two words" as "twoWords" or "TwoWords". This convention is commonly used in Pascal, Java, C#, and Visual Basic. Treatment of initialisms in identifiers (e.g. the "XML" and "HTTP" in XMLHttpRequest) varies. Some dictate that they be lowercase (e.g. XmlHttpRequest) to ease typing, readability and ease of segmentation, whereas others leave them uppercased (e.g. XMLHTTPRequest) for accuracy.
