"The use of double underscores to indicate "magic" words goes at least as far back as the C preprocessor, which used that convention starting in the 1970s (e.g.

"The use of double underscores to indicate "magic" words goes at least as far back as the C preprocessor, which used that convention starting in the 1970s (e.g., __FILE__, __LINE__). It did so because there were no symbols that were off-limits to the preprocessor, so a rarely-used naming pattern (multiple underscores and upper case words) virtually ensured a lack of conflicts." http://programmers.stackexchange.com/a/228340/84451 http://programmers.stackexchange.com/a/228340/84451