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When using algebraic notation, I knew that + means check and # means checkmate. When I first saw ++ sign, I intuitively thought that this should mean double check, because of the double plus. However, if you look here: "http://www.fide.com/component/handbook/?id=125&view=article". Appendix C.13 says that ++ also means checkmate, isn't this a bit confusing? Shouldn't it be meaning double check insted of checkmate(#)??
There have been various ways of noting double check in notation over the years. The old descriptive notation just used "dbl ch" but some book publishers got creative and used § or ‡ or some other symbol. But it never was recognized because under the Laws of Chess, double check is just check.
"++" is checkmate another one is "#"