Adopting the Slink Marking Bypass

This post reviews the origins of the slink marking bypass, now adopted for Sysudoku basic solving. A bypass trace on Very Hard 28 illustrates the challenge available in a basic level puzzle.   It also kicks off a series of  Dave Green’s Labor Day  weekend puzzles  that show off the bypass.

Working through the Beginner’s page and the early posts on basic solving, I came to realize how inevitable it was for me to adopt the slink marking bypass as the way to begin puzzles of any level.  After all, Sysudoku was started when I realized, with due diligence, that human solvers needed a better way to begin.

For a time before that I had wondered what I might be missing, because Sudoku advisers devoted so little attention to finding candidates.  So much effort went into generating candidates, then removing so many of them. There wasn’t even a name for what seemed to be the default starting method.  Tom Sheldon in his Master Class, called it the “completion strategy”.  Not wanting to dignify it that much, I called it “number scanning”.

I was ready to begin when I was convinced that box marking with dublex and crosshatch, then finding the remaining candidates with line marking,  finds all remaining candidates more efficiently with much less distraction.  I’ll tell you how it started, but first, you may have more work to do on Very Hard 28.

vh-28-basic-trYour assignment is to inspect this Very Hard #28 basic checkpoint trace.  Unless your trace looks better, you must read through it, marking your copy of the grid, and decide exactly what each move is and exactly why it is possible.

For navigation hints, see the traces page.



The slink marking bypass originated in my review of Peter Gordon’s basic solving advice in his Guide to Sudoku Solving.  His basic solving I recognized as box marking with clues only, with limited line marking.  While definitely not enough, it was a positive step toward minimizing clutter.  Although he did not explain them explicitly, Peter was evidently tracking some slink effects and naked pair claims on cells.  I decided to try this as a first stage of box marking, with pencil marking for naked pairs only.  I called it the dublex bypass, and recommended it for speed tournaments, in October 2014.

Shortly afterwards, I was doing  basic clinic posts, reviewing some Dave Green puzzles.  So  I compared traces with the bypass and without.  The bypass did surprisingly well, finding almost all clues and replacing some slinks with clues.  Slink marking was easier to  accomplish, after the enjoyably challenging bypass.

Next I reviewed Wayne Gould’s Train Your Brain with Su Doku, Fiendish collection . Wayne advocated no pencil marks at all. He doesn’t give any instruction on basic solving, but his basic level puzzles do require taking slinks into account.  See the review table and quotes in the 11/04/14 post.   I worked through them while trying to track naked pairs mentally, but gave up on this memory exercise.   Not because they don’t work.  I just don’t like to do them, even though I should.

Since November 2014 I’ve been using the bypass effectively.  It definitely takes slink marking further in minimizing distracting clutter of less than relevant candidates.  I think it also adds the challenge Wayne had in mind.

Clues are discovered in line marking, often the triggering a collapse.  But clues found in my box marking all too frequently reveal something overlooked in the bypass.  Keep that in mind when you read a bypass trace in Sysudoku.  After a tough-to-diagnose effect, you might wonder how you could have seen it.  Well, I probably didn’t.  However, discovering what I’m missing in the bypass, I believe I’m seeing more.  Thanks, Peter and Wayne!

Next we’ll  checkpoint our way through the Dave Green Labor Day weekend trio.  I think it will show you another side of Dave’s excellent basic series.  I was sure one of these would cross the Wizard Green’s line into advanced solving.  But it didn’t happen.   Dave knows what he’s doing.

green-9-02Here is Green’s **** of Friday, September 03, 2016. The bypass starts in the NE.  I’m counting on you to figure out why and where, and to go on from there.  Enjoy!

About Sudent

I'm John Welch, a retired engineering professor, father of 3 wonderful daughters and granddad to 7 fabulous grandchildren. Sudoku analysis and illustration is a great hobby and a healthy mental challenge.
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2 Responses to Adopting the Slink Marking Bypass

  1. Gerald says:

    Hello John,

    I was working through the trace for the blog entry “Adopting the Slink Marking Bypass –
    Posted on September 13, 2016” (, using the puzzle Very Hard 28 from the previous week’s blog entry “Mike Peterson’s Very Hard Sudoku, v.1 – Posted on September 6, 2016”. The trace did not make sense to me as it shows NE5 on the second line; but NE5 is a given in the puzzle posted on September 6th.

    I then looked at the puzzle for “Green’s **** of Friday, September 03, 2016” posted at the end of the blog for September 13, 2016. It is the same puzzle as the Very Hard 28. They also have similar file names.

    Is there a different puzzle for Very Hard 28?

    I got here because I was working through the blog entry “A Bypass 3-Fill on the Bus
    Posted on April 11, 2017” and its trace. I found I needed to refresh my knowledge of tracing and the bypass.


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