Lace weight yarns are very fine! This category or weight of yarn is often used for shawls and the needles/hooks used with it (to create a lacy garment) are often larger which makes a standard gauge (number of stitches per inch/2.5 cm) harder to determine. Gauge may vary widely from project to project. For crocheters, 10-count crochet thread would be included in this category.
Knit Gauge (stockinette): 33 - 40 sts/10 cm on 1.5–2.25 mm knitting needles
Crochet Gauge (double crochet): 32–42/10 cm on 1.4 - 1.6mm (steel) and 2.25mm regular crochet hook
For more information about yarn gauges and classifications, visit the Craft Yarn Council website.