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Programming conventions
Underscores inserted between letters are very common to make a "multi-word" identifier in languages that cannot handle spaces in identifiers. This convention is known as "snake case" (the other popular method is called camelCase, where capital letters are used to show where the words start).
An underscore as the first character in an ID is often used to indicate an internal implementation that is not considered part of the API and should not be called by code outside that implementation. In Dart, all private properties of classes must start with an underscore; this usage is also common in other languages such as C++ even though those provide keywords to indicate that members are private. It is extensively used to hide variables and functions used for implementations in header files. In fact, the use of a single underscore for this became so common that C compilers had to standardize on a double leading underscore (for instance __DATE__) for actual built-in variables to avoid conflicts with the ones in header files. PHP "reserves all function names starting with __ as magical."Python uses names that both start and end with double underscores (so called "dunder methods", as in double underscore) for magic members used for purposes such as operator overloading and reflection, and names starting but not ending with a double underscore to denote private member variables of classes which should be mangled in a manner which prevents them from colliding with members of derived classes unless the classes have the same name (__bar in class Foo will be mangled to _Foo__bar). By convention, members starting with a single underscore are considered private or protected, although this behavior only has inherent effect for modules, where import * statements by default import all names that do not start with an underscore, unless an export list is explicitly defined by the module.
A variable named with just an underscore often has special meaning. $_ or _ is the previous command or result in many interactive shells, such as those of Python, Ruby, and Perl. In Perl, @_ is a special array variable that holds the arguments to a function. In Clojure, it indicates an argument whose value will be ignored.In some languages with pattern matching, such as Prolog, Standard ML, Scala, OCaml, Haskell, Erlang, and the Wolfram Language, the pattern _ matches any value, but does not perform binding.Manuscripts
Depending on local conventions, the following kinds of underlines may be used inline on manuscripts to indicate the special typefaces to be used:
single dashed underline for stet, 'let it stand', proof-reading mark cancelled.
single straight underline for italic type
single wavy underline for bold type
double straight underline for SMALL CAPS
double underline of one straight line and one wavy line for bold italic
triple underline for FULL CAPITAL LETTERS (used among small caps or to change text already typed as lower case).
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