user7229: Is a chess program without the ability to promote to knight unacceptably incomplete?
I have a chess program that implements most basic chess rules, such as en passant and castling. However, promotion automatically chooses a queen, as introducing a decision point there would complicate everything from the AI to the replay system to keybindings.
Is a chess program without promotion to knight just no good, or is the average chess player not likely to ever notice that it's missing? And if they do, would it be similar to "oh, no promotion to knight I guess" or more like "how in the world did someone think this was okay? I bet this wasn't even developed by a chess player!" (a safe bet)? I'm hoping that it's trivial enough to be overlooked, but if it really is important then I'll just have to figure it out.

user7243: While under promoting to a knight is relatively rare, under promoting to a rook is not.
It's not uncommon to promote to a rook in a king vs king and pawn endgames to avoid stalemating the king.  

user2013: Maybe it's a bit obscure, but how will you check your move generation with promotions? I mean, for testing your move generator, as soon as a promotion happens, you won't have a way to check whether your promotion code is working properly.
Just in case somebody has no idea what I'm tlaking about;
http://chessprogramming.wikispaces.com/Perft+Results

user4573: Underpromotion may be uncommon but it is important in the sense that, on the rare occasions you need it, you really do NEED it.
The two basic scenarios, covered in the other answer, are:

Underpromotion to non-queen to avoid stalemating the opponent
Underpromotion to knight because your next move absolutely needs to be a check

Neither of these situations warrants an "oh, no promotion to knight I guess" response, as if they're being over fussy and should just choose the queen like everyone always does because its the best. The queen isn't the best here and being forced to choose it is likely to cost them the game.

user3594: Well I definitely wouldn't put it as unacceptably incomplete, after all it is a very rare event that you're forced to underpromote into a knight, unless you're fooling around.
That said, there are cases in real games, where you actually have to promote knights (again very rarely, but still every now and then you see one of those), because e.g. you have to promote with a check (otherwise you may get checkmated), so if in such scenarios it's only with knight that you can check, then you really need the knight-promotion option. A random example:
[Title "1.e8=N+ Forced knight promotion"]
[StartFlipped "0"]
[fen "3K4/4P3/2qk4/8/8/8/8/8 w - - 0 1"]

1. e8=N+ (1.e8=Q Qc7#)

Here's another one: (match between Tomic vs. Winzbeck)
[Title "Forced N or B promotion else stalemate"]
[StartFlipped "0"]
[fen "3rb2k/2P4p/6pP/p2B1pP1/Pp2pP2/1P2K3/8/8 w - - 0 1"]

1. cxd8=N (1. cxd8=B) (1. cxd8=Q)

On the other hand, it may also happen that if you promote into anything else than knight, you stalemate the position, so again knight-promotion needed. You'll find tons of examples from this chess SE post and links therein.
Whereas if you take online blitz games as example, then most often (for time controls < 5 mins at least) players set their automatic promotion to queen so that in case they're in time trouble and have to promote, they can premove it.
All in all, I would say that you should try to implement it for completeness, and if it turns out difficult for you, then don't worry too much about it, no big deal.
I actually do recall a very fun game between Hikaru Nakamura and Boris Savchenko in the previous world blitz championship, where Nakamura had to underpromote into knight with check in order to avoid a mate in 1, here's the clip, it's only 2 minutes, watch it from the start, but if you just want to see the promotion moment, here it is.

7229: <p>Is a chess program without the ability to promote to knight unacceptably incomplete?</p>

<p>I have a chess program that implements most basic chess rules, such as en passant and castling. However, promotion automatically chooses a queen, as introducing a decision point there would complicate everything from the AI to the replay system to keybindings.</p>

<p>Is a chess program without promotion to knight just no good, or is the average chess player not likely to ever notice that it's missing? And if they do, would it be similar to "oh, no promotion to knight I guess" or more like "how in the world did someone think this was okay? I bet this wasn't even developed by a chess player!" (a safe bet)? I'm hoping that it's trivial enough to be overlooked, but if it really is important then I'll just have to figure it out.</p>
 7243: <p>While under promoting to a knight is relatively rare, under promoting to a rook is not.</p>

<p>It's not uncommon to promote to a rook in a king vs king and pawn endgames to avoid stalemating the king.  </p>
 2013: <p>Maybe it's a bit obscure, but how will you check your move generation with promotions? I mean, for testing your move generator, as soon as a promotion happens, you won't have a way to check whether your promotion code is working properly.</p>

<p>Just in case somebody has no idea what I'm tlaking about;
<a href="http://chessprogramming.wikispaces.com/Perft+Results" rel="nofollow">http://chessprogramming.wikispaces.com/Perft+Results</a></p>
 4573: <p>Underpromotion may be uncommon but it is important in the sense that, on the rare occasions you need it, you really do <strong>NEED</strong> it.</p>

<p>The two basic scenarios, covered in the other answer, are:</p>

<ol>
<li>Underpromotion to non-queen to avoid stalemating the opponent</li>
<li>Underpromotion to knight because your next move absolutely needs to be a check</li>
</ol>

<p>Neither of these situations warrants an "oh, no promotion to knight I guess" response, as if they're being over fussy and should just choose the queen like everyone always does because its the best. The queen <em>isn't</em> the best here and being forced to choose it is likely to cost them the game.</p>
 3594: <p>Well I definitely wouldn't put it as <em>unacceptably incomplete</em>, after all it is a very rare event that you're forced to underpromote into a knight, unless you're fooling around.</p>

<p>That said, there are cases in real games, where you actually have to promote knights (again very rarely, but still every now and then you see one of those), because e.g. you have to promote with a check (otherwise you may get checkmated), so if in such scenarios it's only with knight that you can check, then you really need the knight-promotion option. A random example:</p>

<pre><code>[Title "1.e8=N+ Forced knight promotion"]
[StartFlipped "0"]
[fen "3K4/4P3/2qk4/8/8/8/8/8 w - - 0 1"]

1. e8=N+ (1.e8=Q Qc7#)
</code></pre>

<p>Here's another one: (match between <a href="http://www.chessgames.com/perl/chessgame?gid=1287069" rel="nofollow noreferrer">Tomic vs. Winzbeck</a>)</p>

<pre><code>[Title "Forced N or B promotion else stalemate"]
[StartFlipped "0"]
[fen "3rb2k/2P4p/6pP/p2B1pP1/Pp2pP2/1P2K3/8/8 w - - 0 1"]

1. cxd8=N (1. cxd8=B) (1. cxd8=Q)
</code></pre>

<p>On the other hand, it may also happen that if you promote into anything else than knight, you stalemate the position, so again knight-promotion needed. You'll find tons of examples from <a href="https://chess.stackexchange.com/questions/730/why-would-one-promote-a-pawn-to-a-knight">this chess SE</a> post and links therein.</p>

<p>Whereas if you take online blitz games as example, then most often (for time controls &lt; 5 mins at least) players set their automatic promotion to queen so that in case they're in time trouble and have to promote, they can premove it.</p>

<p>All in all, I would say that you should <em>try</em> to implement it for completeness, and if it turns out difficult for you, then don't worry too much about it, no big deal.</p>

<p>I actually do recall a very fun game between Hikaru Nakamura and Boris Savchenko in the previous world blitz championship, where Nakamura had to underpromote into knight with check in order to avoid a mate in 1, <a href="https://www.youtube.com/watch?v=1-vcBlRNlbQ" rel="nofollow noreferrer">here's the clip</a>, it's only 2 minutes, watch it from the start, but if you just want to see the promotion moment, <a href="https://youtu.be/1-vcBlRNlbQ?t=2m4s" rel="nofollow noreferrer">here it is</a>.</p>
