What the "++" means?

Hi guys, this is my another beginner's question about chess.
What the "++" mark means?
Soma say it means checkmate, some say it means double-check (which isn't always a checkmate)
So, which one?
Thanks, in advance.

if "+" is check, "++" looks like double check, especially since "#" is checkmate.

Is there a double check in the position?

I have seen ++ used to mean checkmate. In either usage it is not standard...

++ used to be check mate. You don't see it that often anymore since most use #. I laughed at ++ being double check.

Wikipedia says:

Check and checkmate

A move which places the opponent's king in check usually has the notation "+" appended. Or sometimes a dagger is used: "†". Or the abbreviation: ch. Double check is commonly notated the same as check, but is sometimes represented specially as dbl ch, or in older books as "++". The Encyclopedia of Chess Openings omits any indication of check.

Checkmate at the completion of moves can be notated as "#" (some use "++" instead, but the USCF recommends "#"). Or the word mate is commonly used. Occasionally the double dagger is seen: "‡".

@OP: ivandh is correct, as to the symbol's usage. Most publications these days, as far as I've seen, use ++ to mean double check. This is a meaningless distinction, though, and monographs only use it trivially, merely to enlighten the reader to the fact that a double check has arisen in the position. Beyond this, double check does not matter: a check is a check, regardless of whether it's "double." That is why Chess.com does not use ++ in the move lists whenever a double check occurs.

As far as I can remember, ++ also used to mean checkmate, which also was already stated above. Indeed, this was how I was originally taught to write a checkmating move.

In MCO-10, + meant a distinct superiority and ++ meant a winning advantage.

I haven't seen that usage in a long time.

I had a wierd thought about double check... I have never seen that in notation...

# this is checkmate....i am gonna guess double check for the ++

double plus ungood

I have always used ++ for double check, and # for checkmate. It's fairly good notation I think.

which, like all double negatives, means really really good.

Orwell wasn't too specific, so we need to take the meaning out of the context.

++

according to page 218 of the US Chess Federation's Official Rules of Chess 5th Edition  http://www.uscfsales.com/product_p/b0012rh.htm

the "++" means checkmate.

What do they say the notation is for double check?

Nah, those were what I mean.

So?

++ is checkmate, + is check or double check

the book doesn't give a notation for double check.

huh?

um, there was discussion going as to whether "++" meant double check or checkmate.  I just thought I'd see what the official rule book for the USCF says about it and post that."     btw, the USCF is the governing body of chess in American and they are affiliated with FIDE, the governing body of chess around the world.