user14257: In my area, most kids including me grew up with a very popular house-rule: After all the pieces of either of the players are removed besides the king, the opposing player has 18 moves to checkmate him. If he can't do that, the game is a draw.
The rule is popularly called 18-moves.
I have spent some time googling and have not found this rule anywhere.
Does anyone know a source for this rule and why it's so popular?

user20721: It is a local rule and is not that popular outside a few small areas.
In Spain there was a 20 move rule.  
THE rule is 50 moves, per the laws of chess,  without any captures or pawn moves.  In special cases more moves are allowed because of the difficulty of mating. One example is the wKKN vs KP endgame in which one knight blocks the pawn from moving while the remaining KN corner the other king then the other knight starts moving to give mate before the black pawn can queen and start checking. 

user14697: I had one of like it too but it consisted of 20 moves. The most similar thing officially is the 50 moves rule, where if a pawn hasn't moved and nothing has been captured for the last 50 moves, it's a draw. I think this is the source of many of those house rules in an easier version.

user14778: Clearly these house rules are made for kids and beginners to prevent pointless game stalling, with winning camp promoting 8 knights for instead.of checkmating, etc.. Would be annoying to end up in  KBN vs K with this rule in serious games, when you do know how to win it.

user3890: I grew up in Spain and we used a 20 "move rule" instead of 18.
We played exactly as you said, if a player is left with a single king, the other has to mate in 20 moves or less to win the game. However, this "rule" only applied in friendly matches and was never enforced in tournaments. Our coach made us play like this for several reasons:

It promotes critical thinking at an early stage, where you have to look for faster ways to checkmate your opponent than just queening your two remaining pawns and checkmate using the "staircase" method.
It avoids humiliating the opponent, since now promoting all the remaining pawns to minor pieces and playing around with the king is no longer an option. We were around 6 years old, so once you knew you had won  you wanted to enjoy it longer. This kind of demeaning behavior was discouraged.
It avoids elongating the game more than necessary. We trained without clocks, which could make us spend unnecessary time in trivial positions.

This practice then naturally extended to our friendly games, but we were well aware that this was not an official rule and that on tournaments the 50 move rule applied.

14257: <p>In my area, most kids including me grew up with a very popular house-rule: After all the pieces of either of the players are removed besides the king, the opposing player has 18 moves to checkmate him. If he can't do that, the game is a draw.</p>

<p>The rule is popularly called 18-moves.</p>

<p>I have spent some time googling and have not found this rule anywhere.</p>

<p>Does anyone know a source for this rule and why it's so popular?</p>
 20721: <p>It is a local rule and is not that popular outside a few small areas.
In Spain there was a 20 move rule.  </p>

<p>THE rule is 50 moves, per the laws of chess,  without any captures or pawn moves.  In special cases more moves are allowed because of the difficulty of mating. One example is the wKKN vs KP endgame in which one knight blocks the pawn from moving while the remaining KN corner the other king then the other knight starts moving to give mate before the black pawn can queen and start checking. </p>
 14697: <p>I had one of like it too but it consisted of 20 moves. The most similar thing officially is the 50 moves rule, where if a pawn hasn't moved and nothing has been captured for the last 50 moves, it's a draw. I think this is the source of many of those house rules in an easier version.</p>
 14778: <p>Clearly these house rules are made for kids and beginners to prevent pointless game stalling, with winning camp promoting 8 knights for instead.of checkmating, etc.. Would be annoying to end up in  KBN vs K with this rule in serious games, when you do know how to win it.</p>
 3890: <p>I grew up in Spain and we used a 20 "move rule" instead of 18.</p>

<p>We played exactly as you said, if a player is left with a single king, the other has to mate in 20 moves or less to win the game. However, this "rule" only applied in friendly matches and was never enforced in tournaments. Our coach made us play like this for several reasons:</p>

<ol>
<li>It promotes critical thinking at an early stage, where you have to look for faster ways to checkmate your opponent than just queening your two remaining pawns and checkmate using the "staircase" method.</li>
<li>It avoids humiliating the opponent, since now promoting all the remaining pawns to minor pieces and playing around with the king is no longer an option. We were around 6 years old, so once you knew you had won  you wanted to enjoy it longer. This kind of demeaning behavior was discouraged.</li>
<li>It avoids elongating the game more than necessary. We trained without clocks, which could make us spend unnecessary time in trivial positions.</li>
</ol>

<p>This practice then naturally extended to our friendly games, but we were well aware that this was not an official rule and that on tournaments the 50 move rule applied.</p>
