Things You'll Need
Often knitters choose to knit hats in the round to avoid having to seam the edges together at the end. But if your pattern calls for your hat to be knitted flat on straight needles, you will have to seam the edges together to complete the hat. Seaming the edges of a hat together is easy because you will have an equal number of rows on both sides. Line the rows up with each other and use an invisible mattress-stitch seam to join the edges of a stockinette-stitch hat.
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Line up the edges of the hat with the right sides facing you. Each row should match up with a corresponding row on the opposite edge. Pin the edges together to hold them in place while you sew the seam.
Thread the cast-on tail onto a yarn needle. If you did not leave a long cast-on tail, thread the needle with a new length of matching yarn and draw the yarn through the bottom left corner. Leave a 6-inch tail if you are using a new length of yarn.
Bring the needle under the right edge and draw the yarn up through the corner stitch on the right side.
Bring the needle under the left edge and draw the yarn up through the corner stitch on the left side. Pull the yarn tight. This completes a figure-eight stitch and secures the two bottom corners together.
Pull apart the first and second stitch on the first row of the right side and locate the horizontal bar between the stitches. Insert the needle from bottom to top under this bar and pull the yarn through. Leave some slack in the yarn.
Locate the horizontal bar between the first and second stitch on the corresponding row on the left side. Insert the needle from bottom to top under this bar and pull the yarn through.
Continue looping the yarn under the horizontal bars of each row, going back and forth across the seam, until you reach the top of the hat. Pull the yarn tight every few rows to close the seam while working your way up the hat.
Once all the rows have been joined, create a figure-eight stitch in the top corners the same way you joined the bottom corners.
Pull the needle and the remaining tail yarn through the top of the hat so the yarn is on the inside of the hat. Turn the hat inside out and weave the tail yarn through the stitches on the inside of the hat to secure the yarn.
When casting on, leave a tail 18 to 24 inches long and use this yarn to create your seam.
If your cast-on tail is on the right edge rather than the left edge, reverse the instructions so you are working from left to right instead of right to left.
Seam a garter-stitched hat in the same way as a stockinette hat, except loop the yarn around the purl bumps rather than the horizontal bars on each row.