Plaid Filling Stitch

Isn't this the perfect Autumn colored plaid?  This intricate looking stitch is the Plaid Filling Stitch.  It is also known as the Tartan Stitch.

This is the latest stitch I have learned from my 100 Stitches Plus list.

Although this looks intricate, it is made up of simple Straight Stitches worked in a grid pattern with three layers.
To begin, I selected three different colors of perle cotton floss:

I used a ruler and a water soluble marking pen to draw a grid.  Mine has three horizontal lines and six vertical lines.

I decided to use the brown floss for the first layer, which makes up the outline of the grid.  

I started at the top of the first vertical line.  I came up through the fabric:

 pulled all the way through and went back into the fabric at the bottom of that first vertical line: 

 This formed the first Straight Stitch:

I worked Straight Stitches on the six vertical lines from left to right.  
Next, I came up through the fabric on the bottom horizontal line:

I made a long Straight Stitch on each of the three horizontal lines:

That forms the grid.  The next layers are worked over the grid and provide the plaid pattern effect. 

For the second layer, I used the dark yellow floss.  

This layer is worked only horizontally.  I started on the left edge of the first vertical Straight Stitch. Notice, this stitch will be below the top horizontal Straight Stitch, it will be worked inside the grid: 

 I made a horizontal Straight Stitch across the first two vertical Straight Stitches:

Then the next two and the final two.  Then I worked back from right to left:  

making horizontal Straight Stitches across the same sets of vertical Straight Stitches:

For the next row, I started again on the left edge of the first vertical Straight Stitch, but this time below the second horizontal Straight Stitch.  Also, to make a plaid pattern, this row needs to be offset from the rows above.  To do that, this first horizontal Straight Stitch is worked over only one of the vertical Straight Stitches:

Then over the next two 

and the next two and then over the single remaining vertical Straight Stitch.  
I worked back from right to left to form the next row the same way, going over the same sets of vertical Straight Stitches: 

That completes the second layer:

For the third layer, I am using the red floss.  

The third layer is worked the same way as the second layer only vertically. 
To continue the plaid effect, this layer also needs to be offset from the layer below.  To achieve that - I first came up through the fabric between the first two vertical Straight Stitches and above the top horizontal Straight Stitch:

I made a horizontal Straight Stitch by going back into the fabric below the middle brown horizontal Straight Stitch:

Then made a second Straight Stitch over the single bottom brown horizontal Straight Stitch

I continued these offset Straight Stitches all across the grid:

This is a look at the underside of the stitch:

I learned this stitch from the book Embroidery Stitches by Mary Webb.