A filet crochet chart is a grid, and each square in that grid is one block of the fabric: either an open square (a mesh, usually one double crochet, two chains, one double crochet) or a filled square (typically three or four double crochet worked solid). You read the grid the way you read text — one row at a time, in the direction the row is worked — and the picture appears because filled and open squares sit in different places. That is the whole system. Everything else is bookkeeping.
What makes filet different from most crochet is that you are not following written instructions at all. A doily pattern or a lace edging worked in thread — the kind of thing you would make with a steel hook, say a size 1 at 2.75 mm — is almost always charted, because a written-out row of two hundred stitches would be unreadable. The chart is the pattern. If you can read the grid, you can work any filet pattern ever published, in any language, without translation.
Reading the grid
Establish which direction your rows run before you make a single stitch. Most filet charts are worked in rows back and forth, and the chart is drawn so that row 1 is at the bottom and the last row is at the top — the same way the fabric grows in your hands. The row numbers are printed at the side, alternating left and right: odd numbers on the right edge, even numbers on the left, or the reverse depending on the publisher. That alternation is not decoration. It tells you which end of the row you are starting from.
A filled square is solid fabric; an open square is a hole. On most charts a filled square is shown as a solid block or a square with an X or a dot in it, and an open square is left blank. Publishers vary, so check the key before you start — a chart where the key is missing is a chart you should set aside until you find the source. The other thing to nail down early is the mesh ratio. Classic filet uses a 3-double-crochet mesh: three stitches across and three rows up per square, giving square blocks. Some patterns use a 4-stitch mesh, which produces a slightly wider block, or a 2-row mesh, which squashes the picture vertically. If your finished piece looks stretched, the mesh ratio is almost always the culprit, not your tension.
Count in blocks, not in stitches. Once you know that one square equals one double crochet, two chains, one double crochet, you stop counting individual chain stitches and start counting squares — which is the only way to keep your place across a long row with a picture in it.
Following the picture
Work the chart one row at a time and mark each row off as you finish it. A strip of removable tape, a magnetic board, or a highlighter on a photocopy all work; what does not work is trying to hold your place by memory across a row of thirty or forty blocks. The picture is not a separate thing from the stitches — it is the stitches. When you see a diagonal line of filled squares running up the chart, you are looking at a row of solid blocks offset by one square each row, and that is what makes the line look diagonal in the finished fabric.
Where people go wrong is at the row ends. A filled block at the edge of the chart often needs an extra stitch at the turning chain to stay square, and a chart will not always tell you this — it assumes you know the convention the designer used. If your edges are pulling in or flaring out while the middle of the piece looks correct, the problem is at the turning chain, not in the body of the row. Rip back one row, add or remove the stitch at the edge, and carry on. It is a five-minute fix if you catch it early and a two-hour fix if you catch it twenty rows later.
Reading the chart is also how you spot errors in the pattern itself. Count the blocks in each row against the row above. If row 12 has one more block than row 11 with no shaping indicated, either the chart has a mistake or you have misread the key. Photocopy the chart and mark the discrepancy rather than second-guessing yourself mid-row.
Tools and tension
Thread and a fine steel hook give you the crispest filet, because thin yarn makes small squares and small squares make legible pictures. That is why filet is so strongly associated with doilies and lace edgings — detailed work where the mesh needs to be fine enough that a curve reads as a curve rather than a staircase. If you are working a large pictorial panel and you want the image to be recognisable at a glance, go finer than you think you need to.
The hook size has to match the thread or yarn, and the standard sizes are worth knowing because filet patterns from different sources will name hooks in different systems. A G-6 is 4.00 mm, an H-8 is 5.00 mm, and an I-9 is 5.50 mm in the Craft Yarn Council standard system. A G-6 is a medium hook, commonly used with worsted-weight yarn for beginner-friendly projects like basic blankets and hats; an H-8, at 5.00 mm, is the one most often recommended for learning to crochet with medium-weight yarn and for granny squares and simple blankets; an I-9, at 5.50 mm, is used with medium to light bulky yarns to make a looser fabric for things like baby blankets. None of those are filet hooks in the traditional sense — filet in the classic style is thread work — but if you are adapting a chart to worsted-weight yarn for a blanket, the H-8 is a reasonable starting point and you will size up or down from there based on whether your blocks come out square.
Squareness is the only tension test that matters in filet. Work a swatch of ten blocks by ten blocks, measure it, and check that the width and the height match. If they do not, change hook size, not your grip. Fighting your natural tension to make a chart come out square is a losing battle across a large piece.
Working from the chart
Start with the foundation chain, and count it in blocks plus the turning chain. The most common beginner error in filet is chaining the exact number of stitches the chart shows across without accounting for the turning chain and the edge stitch, which leaves the first row short and skews everything above it. If you have ever learned to start a crochet chain for a plain scarf, the mechanics are identical — the difference is that here the count has to be right to the stitch, because the picture depends on it.
Once the foundation is correct, the chart runs itself. You are not learning new stitches for filet; if you can work a double crochet, you can work filet. The other common stitches — half double crochet, the magic ring, working in a circle, changing colours — come up in filet only at the edges, if at all. A filet heart or a filet motif in a granny-square-adjacent project is usually just a chart placed inside a border worked in ordinary stitches. The chart governs the interior; the border is where you change colours, join rounds, or work a magic ring to start a centre-out motif.
Colour changes in filet are done the same way as anywhere else — at the last pull-through of the stitch before the new colour — but be deliberate about it. A filet chart with two colours is really two charts overlaid, and the block boundaries are where the change happens. Change mid-block and you get a ragged edge to the picture that no amount of blocking will fix.
The honest limitation of chart-reading is that it does not tell you how the fabric will behave. A chart is a plan for where the holes go. Whether the finished piece hangs straight, whether the mesh opens up after blocking, whether your thread has enough body to hold the shape — none of that is in the grid. That part you learn by making the swatch, and by making the second one after you have blocked the first.