To create a filet crochet pattern for a crawling baby that is 42 squares long and 31 squares tall, you will need a crochet hook size US D/3 (3.25 mm), UK 10, and Metric 3.25 mm. Use lightweight yarn in size 2 (Fine) or 3 (Light) in the following quantities:
Pattern:
Color A = O, Color B = X
Row 1-31: Chain 42 with Color A, turn
Row 2: 1 DC in 4th ch from hook, *ch 2, skip 2 ch, 1 DC in next ch; repeat from * across, turn
Row 3: Ch 3 (counts as 1 DC), *ch 2, 1 DC in next ch-2 space; repeat from * across ending with 1 DC in top of turning ch, turn
Row 4-30: Repeat Row 3 alternating colors every 2 rows (Color A for even rows, Color B for odd rows)
Row 31: Repeat Row 3 with Color A
Finish off and weave in ends.