Traditionally filet crochet charts are presented as a set of the open and solid blocks. Open and solid blocks create a filet crochet mesh. Filet crochet mesh can be crocheted in many different ways.
The most often used filet mesh usually consists of double crochets and chains between them. For example, so called 4-dc mesh has four stitches in each block. Solid block consists of four double crochets: dc, dc, dc, dc. To make open block we would substitute two middle double crochets by two chains: dc, ch 2, skip 2, dc.
This is the way our filet crochet cat was made. Let's compare a chart, to crochet a cat, and final item below. As a rule, filet crochet charts are graphed on a grid where each cell is a SQUARE. For each real cell/block to remain a square, its horizontal and vertical sizes have to be equal.
For traditional 4-dc mesh vertical dimension of each block is equal to the height of ONE double crochet. While its horizontal dimension is determined by the length of two chains between two double crochets . In other words, to keep our blocks squared, length of two horizontal chains has to be equal to the length of one vertical double crochet.
Let's look what happens in reality. I made two swatches 10 blocks x 10 blocks each. Swatch # 1 is traditional 4-dc mesh. Measurements show that our 10 x 10 swatch is NOT a square. Its horizontal dimension is larger than vertical. That's why cat's body (still being quite cute) looks somewhat horizontally elongated if we compare it to the chart.
In many cases this kind of distortion is not really important. We traditionally continue to graph our filet crochet charts on a grid with square cells. What if your beautiful round doily pattern, for example, turned out to be an oval once you've finished it ? What if you do not like the pattern was shrunk in vertical direction because real blocks are not really squares?
What is a solution? To make them squared. -:) One of the simplest ways to do so is to use another type of double crochets which are taller than regular double crochets. We call them EXTENDED double crochets (Edc).
Swatch #2 is crocheted with Edc and looks like practically ideal square. Your results can be a little different size wise, but Edc is certainly a better alternative when it is necessary to keep square shapes for the blocks , to avoid disproportion in your pattern.
I would like to introduce one more not that well known filet crochet technique. I always liked filet crocheting. It is very simple technique, looks great in different designs being just plain filet crochet, but at the same there are so many ways to make it even more expressive and bright.
Our cat is one of those filet crochet designs where some additional color will not heart general impression. So... We have a plain cat crocheted on 4-dc mesh. We are going to use contrast color yarn (experiment with thickness if you wish) and crochet single crochet edging around contour created by the solid blocks.
To crochet edging, insert your hook into the tops of double crochets on the horizontal lines of the contour. Vertical lines are outlined by the bodies of double crochets. Perhaps three single crochets are necessary to go along each dc. Here they are! Before and after. The very same project. Just different light when I took pictures. Of cause I could not resist and added "real" eyes to it. -:)
To make your filet crochet design brighter, it is not always necessary to use contrast color to highlight contour. Solid blocks on mesh look very elegant if you go around them with the same or very close to the filet mesh color.