SUDOKU #12  
 

I'm going to repeat the info at the top of Sudoku #1, just in case you haven't seen that one yet.
 
I've been doing Sudoku for a while. There are a lot of people like me who have figured out some tricks or techniques that help them complete the puzzles. This page is not really for them. I met a woman recently who was just starting to try to do Sudoku. This page is for people like her. I'm not saying that I know every single trick, but I know enough to complete most puzzles.
 
The basic rules of Sudoku are that there is only one of each number in each row, column, and 9 square grid. There isn't any mathematics or arithmetic. If you know how to count from 1 to 9, then you can do these puzzles. All of the techniques are based on this rule.
 
Here is a sample Sudoku puzzle.
 
    9   7           6  
      2   1         8  
    1       7   4 9    
        9 4            
      3           7    
              8 3      
      1 5   6       9  
    3         5   1    
    2           6   3  
 
This is an Evil difficulty puzzle (harder than Hard), and it will allow me to show some examples of a few of the tricks that I've uncovered. I've added the letters below to the rows and columns in order to talk about each of the 81 squares. As a convention, I will talk about squares and boxes. There are 81 squares, but there are 9 boxes of 9 squares.
     
  A 9   7           6  
  B   2   1         8  
  C 1       7   4 9    
  D     9 4            
  E   3           7    
  F           8 3      
  G   1 5   6       9  
  H 3         5   1    
  I 2           6   3  
  J K L M N O P Q R  
                         
There is a 7 in BP (because of AL, CN, and EQ).  So there is a 7 in HR.
         
  A 9   7           6    
  B   2   1     7   8    
  C 1       7   4 9      
  D     9 4              
  E   3           7      
  F           8 3        
  G   1 5   6       9    
  H 3         5   1 7    
  I 2           6   3    
    J K L M N O P Q R    
                             
There is a 9 in EP (because of CQ, GR, and DL).  There is a 1 in AP (because of CJ, BM, and HQ).
             
  A 9   7       1   6    
  B   2   1     7   8    
  C 1       7   4 9      
  D     9 4              
  E   3         9 7      
  F           8 3        
  G   1 5   6       9    
  H 3         5   1 7    
  I 2           6   3    
    J K L M N O P Q R    
                         
There is a 5 in IQ (because of GL and HO), a 5 in CR (because of IQ), and a 5 in DP (because of CR and IQ).
         
  A 9   7       1   6    
  B   2   1     7   8    
  C 1       7   4 9 5    
  D     9 4     5        
  E   3         9 7      
  F           8 3        
  G   1 5   6       9    
  H 3         5   1 7    
  I 2           6 5 3    
    J K L M N O P Q R    
                         
There is a 2 in AQ (because of BK), so there is a 3 in BQ.
         
  A 9   7       1 2 6    
  B   2   1     7 3 8    
  C 1       7   4 9 5    
  D     9 4     5        
  E   3         9 7      
  F           8 3        
  G   1 5   6       9    
  H 3         5   1 7    
  I 2           6 5 3    
    J K L M N O P Q R    
                         
There is an 8 in DQ (because of BR and FO).  The numbers missing from column Q are a 4 and 6, but there is a 6 in GN.  So the 6 is in FQ, and the 4 is in GQ.  There is a 3 in CL (because of HJ, EK, and BQ).
         
  A 9   7       1 2 6      
  B   2   1     7 3 8      
  C 1   3   7   4 9 5      
  D     9 4     5 8        
  E   3         9 7        
  F           8 3 6        
  G   1 5   6     4 9      
  H 3         5   1 7      
  I 2           6 5 3      
    J K L M N O P Q R    
                         
There is a 1 and a 2 in EL and FL (because of CJ, GK, IJ, and BK).  So there is an 8 in EJ (because of DQ and FO).
         
  A 9   7       1 2 6    
  B   2   1     7 3 8    
  C 1   3   7   4 9 5    
  D     9 4     5 8      
  E 8 3 12       9 7      
  F     12     8 3 6      
  G   1 5   6     4 9    
  H 3         5   1 7    
  I 2           6 5 3    
    J K L M N O P Q R    
                         
There is a 4 and a 5 in FJ and FK (because of DM and DP), so there is a 6 and a 7 in DJ and DK.
         
  A 9   7       1 2 6    
  B   2   1     7 3 8    
  C 1   3   7   4 9 5    
  D 67 67 9 4     5 8      
  E 8 3 12       9 7      
  F 45 45 12     8 3 6      
  G   1 5   6     4 9    
  H 3         5   1 7    
  I 2           6 5 3    
    J K L M N O P Q R    
                         
There is a 4 in ER (because of DM and FJ-FK).
         
  A 9   7       1 2 6    
  B   2   1     7 3 8    
  C 1   3   7   4 9 5    
  D 67 67 9 4     5 8      
  E 8 3 12       9 7 4    
  F 45 45 12     8 3 6      
  G   1 5   6     4 9    
  H 3         5   1 7    
  I 2           6 5 3    
    J K L M N O P Q R    
                         
We can treat FJ and FK as solved even though we don't know which square the 4 and 5 are actually in, so the number missing from row F are the 1, 2, 7, and 9.  There has to be a 7 in FM (because of EQ-HR, DJ-DK, and CN), and there is a 9 in FN (because of DL and GR).  There is a 1 and a 2 in EL and FL (which we've already established), but now we know that there is a 1 and a 2 in DR and FR.
         
  A 9   7       1 2 6    
  B   2   1