Most of these curtains are crocheted across, back and forth in narrow rows, with the straight edge at the top of the window on one side, and increasing and decreasing to the points on the other. This way, they can easily be made to fit the width of the window.
And most of them use a technique called ‘filet crochet’, sometimes in combination with other techniques. Filet crochet consists of a kind of grid made up of chain stitches and what is known in the US as double and in the UK as treble crochet stitches. Some of the squares are left open and others are filled with double/treble crochet to make symmetrical shapes, flowers or even intricate pictures.
The ones above are 100% filet crochet, whereas the house below has simple narrow ones…