Hello and welcome to this course on type 1 locked candidates!
The locked candidate technique is a technique of elimination that allows you to logically determine which numbers can be eliminated.
To illustrate this technique, we will present two practical cases from real games.
With the help of note-taking, it is possible to note down, for each cell, all the probable numbers. Note-taking is essential, even compulsory, to complete sudoku grids of average level and more. It is with this technique that one is able to proceed by logical elimination and to remove possibilities. If in a box, all the candidates of a specific number are assigned to a single row or column, this number cannot appear outside this box, in the same row or column.
In the case below, we have noted all the cells where the number 5 is likely (the notes highlighted in red).
In cell number 8, we see that there are only two cells that can contain the number 5.
Logically, then, it is impossible for cell R8-C2 to contain the number 5.
Therefore, only cell R7-C2 remains as a candidate for the number 5.
By proceeding by elimination in this way, it is possible to remove locked candidates and find numbers very simply.
This time, we will use this technique on the columns and not the rows as before. Here, we are looking to use this technique on the number 8.
The whole C3 column is locked and we can no longer add 8 to it. So we are left with only column C1 and C2. In box number 4, we see that there are only two candidates for the number 8 and that these candidates are on the same column.
In this case, as in the previous example, it is therefore impossible for the number 8 to appear the cells in column C1 belonging to box number 7.