Sysudoku Home Page

Systematic Sudoku (Sysudoku) is a human engineered way to solve Sudoku puzzles, and provides graphic and completely documented examples and collection reviews at all levels. Diagrams and tables that aid solving are demonstrated.

 Over 500 weekly posts give complete examples with move by move traces for you to follow, and detailed diagrams of highlighted moves. Actively reading traces, by filliing your own grid as you anticipate, then verify each move, is an excellent way to discover and appreciate new techniques, and new situations.

The Menu Bar accesses the site elements are described below.

Latest Updates:

The Monthly Archive

Sysudoku posts can be accessed by scrolling back through posts, but individual posts are directly accessed by date.   Click on a month in the right side column under Archives and the posts of that month are shown with date, title and an opening phrase. Click to read the post. Comments and replies for the post are at the bottom of the full text. Categories and tags are also listed at there. Click on a tag for a list of posts with the same tag. Use the back arrow to close the display and , return to the month’s page for browsing. Another back arrow and you’re back to the original post, usually the home page.

Search Box

A Search box is at upper right on the post window. It brings a list of posts related to your search term, similar to the archive or tag list.

Menu Bar

The menu bar under the header picture links to the parent pages described  below. Click for the parent page, or drag down rightward for the menu for child pages.

About Sudoku

These pages briefly describe the distinctive Systematic Sudoku approach to solving puzzles. A walk through demo of Sysudoku Basic is included. . If you’re new, start with About Sysudoku


The page pyramid manual on Sudoku methods and Sysudoku innovations.


Click for the parent page that describes the contents of a review table. The review pages are included on some weekly posts. Drag for a list of reviews and dates, then use the monthly archive to get the review post.

Solving Tools

This is information on solving aids, including the use of  Microsoft ©PowerPoint and ©Word templates for solving, how to load a puzzle and how to draw curves, and coloring technique.

Sysudoku Speak

A glossary of Sysudoku and related Sudoku community terminology.

Sysudoku Traces

An explanation of follow up traces, which is probably unnecessary for experienced solvers.


This page lists post titles, organized by collection or reviewed author. It’s handy for browsing and includes dates for monthly archive access.

Comment on this page if you if you have navigation questions or suggestions.

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Closing With the Hidden Dublex

This closing post adds two examples of a fierce looking line marking being de-fanged by hidden dublex on the over abundant X values

A double line exclusion, or dublex, is one of the primary moves of Basic sudoku, in which the intersection of two parallel lines through a box cannot contain a candidate of the dublex value, forcing a third line chute to contain a true candidate of the value.

When each of the intersecting lines only contain candidates of the value, then each contains a true candidate.  We just don’t know which line remainder contains which true candidate. We call that a hidden dublex, or in trace language, an hdx.

The hidden dublex is a frequent move in the bypass and follow up, but is especially useful when an over abundant value needs weeding. Scroll down for the givens of Extreme Hard 1503, with one 7-candidate and no sign of the 8 value.

The bypass leaves 4 3-fills, each with an 8 candidate to be placed.

On the 7 value of box marking, columns 4 and 6 contain true 7 candidates outside of the C box, for a 7-slink, and soon after SW7m generates the second slink W7m for a 7-wing, another form of hidden dublex.

The 7-wing’s E7m slink is trimmed to clue E7 by the second hdx in c79, squeezing c8 to the naked pair 18, for a 1-wing keeping 1 out of r5c9. But the real damage is done in the 8 camp as C8m starts a collapse.

The collapse trace:

 I have one more Extreme Hard example of the hidden dublex avoiding a tough line marking. It’s 1506.

At the 9 value in box marking, note the hdx on r8 which creates slink W9m. The  hdx chain continues with a c2hdx forcing NW9!

The immediate collapse is typical of unbalanced puzzles tripped up when headed for a cluttered line marking.

To illustrate, here is the line marked grid when the hidden dublex chain is ignored. The 8 and 9 candidates are everywhere.

Weekly posts pause now, awaiting reader requests. Next week, expect the home page, where update announcements and comments will continue. The pause will allow the pages and over 500 posts to be brought to the level of graphics and instruction realized my 12 years of Sysudoku blogging.

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Bring Your Boxline for Extreme Hard 1505

I was closing down the weekly posts with a set of scroll down basic level puzzles, when I got another collection book , 3000 more “very hard to extreme “, for Adult puzzles. I did a few from the extreme section, and found them to be generally bordering on advanced level. Many Extreme Hard puzzles start with a value or two missing from the givens, and consequently, will have at least one very poorly restricted value on the line marked grid. It’s a situation calling for boxlines and X-wings to attack those over abundant candidates. 

In the Guide, a boxline is explained as a feature of the bent region, an intersection of a box and a crossing line. The cells in the box, but outside the line, is the box remainder. The cells in the line, cut outside the box, is the line remainder.

This picture makes the boxline rule simple to state: if a candidate value is absent from one remainder, it must be absent in the other remainder. That’s because there has to be a true candidate of that value in the intersection, which I sometimes call the chute.

Boxlines work in both directions, and you could have a series of boxlines alternating box and line remainders absent a value. When the chute contains more than one candidate of the boxline value, the boxline becomes a slink or aligned triple pointer. Extreme Hard illustrates both of these. Scroll down for the citation and starting grid for your discovery of this. You’re 18 or older, right?

In the bypass and box marking, values 4 gets no action, and is destined to cover the line marked grid in candidates. In the 3-fills, the 4-candidates are already showing up.

Here is the box marked grid. Missing value 4 is the threat, with almost missing value 3 a potential problem as well.

In 1505, the Wr5 boxline effect  is        

E3 => ( 3absent in r5 remainder) slink W3m.

The NWc1 boxline effect is

W3m (absent W remainder => (absent NW remainder NW3m.

The NW3m slink leaves a hidden single N3 in r1, and a naked pair in N.

As line marking begins, N3 has dismissed the only 4 in r1 outside of NW, leaving NW4m and a naked triple in c1.

The rest is follow up.

Next week, let’s see what you do with a second Extreme Hard, 1503, with one 7 and no 8 candidates. There are 7-wings to help avoid a burdensome line marking.

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An AndrewsMcMeel  6-Star Sample 4-fills Out

Scroll back for the givens grid. After the fourth 4-fill, we never get back to the three unresolved 3-fills in the bypass trace.

Here is the grid, just before SE6 (halfway down) wipes out the naked pairs.

The Sysudoku basic level close down is complete. Many basic level example traces are reached by scrolling back.  For each example, the givens grid is one post further back.

Earlier basic level examples, and advanced examples, can be located by date on Titles, and found by the monthly archive.

The weekly posts are extended to add in some examples of boxline exclusion in Sysudoku Basic.

Here is 1505 from Sudoku Puzzle Book for Adults, Extreme Hard by Markus Mangold, Gingen & Sons, Germany, available on Amazon. Note there’s one 3 and no 4 givens. Dust off the boxline page in the Guide. Boxlines, including slink and aligned triple pointers, help in unbalanced puzzles of this type.

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A Filling USA Today Weekend 5-Star

Starting with a 2-fill, two 3-fills, and a 4-fill near collapse, there’s value by value action left in the bypass for only one value. Scroll back for the givens and trace out the bypass.

An x-fill lists the values to be filled in the line. The square bracketed effects are the clues and subsets placed in the line

Here’s the grid you take into box marking.

Box marking adds all candidates which are members of box slinks, strong links due to having exactly two of the same value in a box. Box slinks are marked in the same position at the tops of cells.

Some slinks are already marked. They are subsets, naked pairs, two values in two cells in a box or line. Column 3, c3, has a line slink.  Its position is bottom right because the line is a column.

You can follow this trace on your same grid into box marking. The box marking trace is a list of boxes having box slinks for each value. There is no list for 5 or 8 because all value 5 and 8 clues are placed. The naked pair 69 in the NE box is the result of the value 9 slink combining with the earlier 6 slink, and the pair overwrites a 1 candidate, placing the other 1 in the NE box.

The follow up is fatal, but tracing is suspended because order of placements no longer matters.

Here is the grid at that point, when many naked pair subsets remain.

This week after x-fills, only the value 8 remained to complete the bypass. The week before that, the Kampelmann Hard 109 bypass reached a solution on x-fills alone. I was waiting for the next Washington Post 6-star when my Sudoku expert pal Gordon Fick suggested I look at the WP puzzle supplier AndrewsMcMeel’s website. Turns out, the site has a sample 6-star. What could be more typical of their 6-stars, for a last weekly post?  So here it is. Do the x-fills first.

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A Kampelmann Hard With Bypass Coloring

Ever have a Sudoku fail to tie down a solution? Hard 109 of last post seems to fail to do so without a trial. Coloring, which has found many multiple solutions, defines the unique solution for this one.

Starting with 4-fills, r2 has  no clue seeing two box cells.   But c2 places NW2 and leaves a 3-fill for later.

Then in c6, we get a 4-fill bonus. It’s not practical to look for naked singles in every cell, but as you check a 4-fill you already have five values seeing the four cells. Does any cell have three more? Three more see r1c8, but only  5 is absent from c6. Two absentees see r4c6, and two see r8c6, but r6c5 has three more!

Because unresolved 4-fills are so few in number, it’s worth it to scan their free cells for naked singles.

Now on your own or following the trace, the bypass continues:

Eventually you get a near BUG, but with two cells of three values. What is the solution? By trial, one works and one doesn’t.

Instead, we add coloring. Ignoring column 9, the cluster wraps green, with two green 9’s forced in r8.

The solution is the blue army, plus an uncolored 9r8c9.

Waiting for the next Washington Post 6-star, I went to the library and copied this USA Today Weekend 5-star. It’s almost bypassed by 2, 3 and 4-fills, and holds out through box marking. Well, almost.

Do the x-fills first. It’s the low hanging line marking fruit available in the bypass.

If you’re new, look in The Guide/Bypass for x-fill finding.

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A Washington Post ****** Basic

Scroll back one for the reason for the change in plans, but this is the first of several weekend puzzles from the Washington Post, added at the last minute for additional illustrations of Sysudoku Basic solving.  

Basic starts with a 2-fill and moves smartly to this point. What happens on value 6?

If you’re reading the trace, you only need to figure out the reason for W26.

Going on, the trace continues with the effect of cause C8 being c6(129). That’s a signal that you could have seen the new 3-fill. But that means there was a 4-fill in c7 before C8. Did we miss it?

No, the values 4, 6 and 7 were all present in c7 before C8 made it a 3-fill.  Resolving 3-fill c6(129, 1 and 9 in r6 places C2, leaving the naked pair 19 to be resolved by N1 and a naked pair 35 in r5.

The bypass finishes on value 9, with np67 adding a np35, and a 3-fill in r5 resolving into three bv.

If you’re reading  the traces, notice how the box 3-fill (W) and the bv refill (r5) are marked. In the box marking trace, the naked pair SW45 leaves a 3-fill, resolved to a clue and pair. SW1m is marking a strong link on 1. The link is marked by “m” inside the trace of a different value, 5. Next in this trace, 6 and 9 in r1 force the 3-fill r8[369] and NE7.

Now we’re ready for line marking. It requires care to avoid using candidates covered by unmarked lines. It helps to designate bv where they are really defined.

The collapse on the second line is quick,

but building the cluster to a wrap of blue is even easier.

For next week, we’ll throw in Kampelmann Hard 109 that turned out to be much worse than hard. See if you get far enough to see what I mean.

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A Harder Kampbelmann, Barely Basic

Hard 143 starts with a stingy bypass, and requires box marking and some line marking.

In the bypass, a r9 3-fill resolves  into SW8and the naked pair 36. W8 then follows, either from the 8’s dublex, or  from the West box 3-fill’s naked pair W12.

The full Basic trace shows three clues in the bypass, and one in box marking. The box marking trace identifies the boxes in which box slinks, and line triplets (“t”) occur, for each value. In the box marking event NE3m, the “m”, standing for slink “marks”, is added because the slink occurs on the 6: list, rather than the 3: list.

Each trace follow up is a left to right sequence of nested lists of effect, indented below their causes. This bypass trace here begins with two resolve 3-fills. After resolved n-fills, bypass  traces normally start with clues for  values 1 through 9.  Next is the box marking trace, a  list by value of box slinks (strong links).

The line fill list identifies filled lines at each level of free cells. In this case, two lines with 3 free cells, is followed by five lines with 4 cells, then two lines with 5 cells. The follow up trace leaves an unfilled line of 3 cells, and two with 4 cells.

The trace reaches a stage where the follow up is obvious without a trace, marked by . . .

The bar menu links to a page of explanation of traces, but the best way to learn how to read them is to decide what effect you would mark after each cause, and then note how and where it is described in the trace. At that level, we all think alike.

 I filled in the solution from there with pencil marks, but some of the pencil marks  are effects that never got to be causes. 

Next week’s post was going to be the home page, with navigation information, and a brief report on recent updates in the blog. But on a family visit I discovered that the Washington Post now has what they rate as 6-stars on Saturdays. I’m not talking about the puzzles in the interactive games section, but the Andrews McMeel Syndication puzzles you do on newsprint or on your PowerPoint template.  So I’ll defer closing weekly posts to include several from the Post. Here is next week’s 6-star, from last Saturday, 6/4/22.

Is there another weekend newspaper series that you’d like to sample like this, before Tuesday posts are suspended? Send grids of givens with dates and newspaper identification to, and I’ll squeeze them in to your credit.

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Naked Pairs Carry the Ball in Hard 134

A second Kampbelmann Hard is tough on paper, but  gives way routinely to a systematic bypass.

Here is the grid well into the bypass, with a large number of cells held in reserve by box filling naked pairs.

In the trace, we’re at value 9, just before r5[258] is resolved.

The rest is routine follow up.

Next week, the third and last Kampbelmann,  Hard 143, is harder, carrying into line marking, after giving an opportunity to show a box 3-fill (West) in the trace.

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A Hard, But Bypassed Kampbelmann 50

My Basic bypass starts with a resolved 4-fill. Did yours? 

The final collapse begins with the value 8 scan resolving the naked pair left from the resolution of 3-fill c2[258].

In the bypass trace, you can discover when the previous 8 clues were found.

That was a very productive 4-fill. Here is the collapse trace, and solution.

Next week, another Kampbelmann Hard, 134, offers a 3-fill among the givens. Take it first. If it does not resolve immediately, you at least get a fill string to help later.

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