user9841: The long algebraic notation (lan) for null move is usually 00-00 or simply 0000. Chess engines that support the nullmove: "give up a turn, and let the other side move" typically use this as its part of UCI. What  would the SAN representation be? 

user9926: Νικόλαος Μανωλακος asked:

What would the SAN representation [of a null move] be?

Several possibilities present themselves as to what the SAN notation can be, but none that I have found claim to be the official standard specification for SAN. The following results indicate within the quotation marks the annotation used (intended to be used without the quotation marks).

NAG (Numeric Annotation Glyph) "$0"
     IAW art. 10, table 1, line 1 [ref. 1, 2, 3]
PGN's SAN:  "--"
     [ref. 4, 5, 6, 7]
     Note:  In ref. 7, null moves start out as mere comments, and can become [null] moves, but it's unclear as to whether they necessarily become so.
PGN's SAN:  "@@@@"
     [ref. 8]
"<>", "--"
     [ref. 9]
     Note:  "A Null Move acts as a placeholder for an expected move that is not played."
"pass", "null", "@@@@"
     [ref. 10]
Stockfish's SAN:  "null"
     [ref. 11]
"(null)"
     [ref. 12]



References:

http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c10 
https://www.chessclub.com/user/help/PGN-spec 
https://metacpan.org/pod/distribution/Games-Chess/lib/Games/Chess/PGN.pod#NUMERIC-ANNOTATION-GLYPHS 
https://github.com/k2052/chess.rb/blob/master/lib/chess/san.rb 
https://www.cs.kent.ac.uk/people/staff/djb/pgn-extract/help.html#portability 
https://www.gnu.org/software/xboard/whats_new/4.6.0/#tag-D6 
http://www.triplehappy.com/faq.html 
https://www.gnu.org/software/xboard/engine-intf.html#8 
http://www.enpassant.dk/chess/palview/manual/pgn.htm#pgncontnull 
http://hgm.nubati.net/alien.html 
https://github.com/mcostalba/FishCooking/blob/master/src/notation.cpp 
https://github.com/erashid/DON/blob/master/src/Notation.cpp 


9841: <p>The long algebraic notation (lan) for null move is usually 00-00 or simply 0000. Chess engines that support the nullmove: "give up a turn, and let the other side move" typically use this as its part of UCI. What  would the SAN representation be? </p>
 9926: <p>Νικόλαος Μανωλακος asked:</p>

<blockquote>
  <p>What would the SAN representation [<i>of a null move</i>] be?</p>
</blockquote>

<p>Several possibilities present themselves as to what the SAN notation can be, but <i>none</i> that I have found claim to be <i>the official</i> standard specification for SAN. The following results indicate within the quotation marks the annotation used (intended to be used without the quotation marks).
<br><br><br></p>

<p><b>NAG (Numeric Annotation Glyph) "$0"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>IAW art. 10, table 1, line 1 [ref. 1, 2, 3]</i></p>

<p><b>PGN's SAN: &nbsp;"--"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 4, 5, 6, 7]</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>Note: &nbsp;In ref. 7, null moves start out as mere comments, and can become [null] moves, but it's unclear as to whether they necessarily become so.</i></p>

<p><b>PGN's SAN: &nbsp;"@@@@"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 8]</i></p>

<p><b>"&lt;>", "--"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 9]</i><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>Note: &nbsp;"A Null Move acts as a placeholder for an expected move that is not played."</i></p>

<p><b>"pass", "null", "@@@@"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 10]</i></p>

<p><b>Stockfish's SAN: &nbsp;"null"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 11]</i></p>

<p><b>"(null)"</b><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<i>[ref. 12]</i>
<br><br></p>

<hr>

<p><br>
<b>References:</b></p>

<ol>
<li><a href="http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c10" rel="nofollow">http://www.saremba.de/chessgml/standards/pgn/pgn-complete.htm#c10</a> </li>
<li><a href="https://www.chessclub.com/user/help/PGN-spec" rel="nofollow">https://www.chessclub.com/user/help/PGN-spec</a> </li>
<li><a href="https://metacpan.org/pod/distribution/Games-Chess/lib/Games/Chess/PGN.pod#NUMERIC-ANNOTATION-GLYPHS" rel="nofollow">https://metacpan.org/pod/distribution/Games-Chess/lib/Games/Chess/PGN.pod#NUMERIC-ANNOTATION-GLYPHS</a> </li>
<li><a href="https://github.com/k2052/chess.rb/blob/master/lib/chess/san.rb" rel="nofollow">https://github.com/k2052/chess.rb/blob/master/lib/chess/san.rb</a> </li>
<li><a href="https://www.cs.kent.ac.uk/people/staff/djb/pgn-extract/help.html#portability" rel="nofollow">https://www.cs.kent.ac.uk/people/staff/djb/pgn-extract/help.html#portability</a> </li>
<li><a href="https://www.gnu.org/software/xboard/whats_new/4.6.0/#tag-D6" rel="nofollow">https://www.gnu.org/software/xboard/whats_new/4.6.0/#tag-D6</a> </li>
<li><a href="http://www.triplehappy.com/faq.html" rel="nofollow">http://www.triplehappy.com/faq.html</a> </li>
<li><a href="https://www.gnu.org/software/xboard/engine-intf.html#8" rel="nofollow">https://www.gnu.org/software/xboard/engine-intf.html#8</a> </li>
<li><a href="http://www.enpassant.dk/chess/palview/manual/pgn.htm#pgncontnull" rel="nofollow">http://www.enpassant.dk/chess/palview/manual/pgn.htm#pgncontnull</a> </li>
<li><a href="http://hgm.nubati.net/alien.html" rel="nofollow">http://hgm.nubati.net/alien.html</a> </li>
<li><a href="https://github.com/mcostalba/FishCooking/blob/master/src/notation.cpp" rel="nofollow">https://github.com/mcostalba/FishCooking/blob/master/src/notation.cpp</a> </li>
<li><a href="https://github.com/erashid/DON/blob/master/src/Notation.cpp" rel="nofollow">https://github.com/erashid/DON/blob/master/src/Notation.cpp</a> </li>
</ol>
