An open mesh on a filet crochet chart is usually indicated with a blank square.
An open mesh in 3-dc filet crochet is made with a chain-1 and a double crochet.
The first mesh of the row is worked over the first 3 stitches of the row but additional mesh are made over only the next 2 stitches (because the last dc of the previous mesh also counts as the first dc of the mesh now being made).
To make an open mesh above a solid mesh: When it is not the first mesh of the row, it is made as follows: ch 1, skip next dc, dc in next dc (as shown in the photo above)
Those abbreviated instructions spelled out: chain 1, skip next double crochet, double crochet in next double crochet
Next ---> How to Make an Open Mesh Above Another Open Mesh --->
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