Two forward slashes making the rest of a line a comment.
Two forward slashes mark a line comment in C and the languages descended from it — C++, Java, JavaScript, Rust, Go — which is most of the languages in common use.
ECMAScript defines a single-line comment as beginning with two forward slashes and running to the end of the line.
Contexts: Software
Inherited from C++ and now the dominant line-comment form; the choice between this and the hash is essentially which language family a file belongs to.
Contexts: Software
Two parallel strokes leaning to the right, set side by side at the height of a line of text.
Both are built from lines and dots, and both are used in software contexts.