Using a tapestry needle and yarn there are two ways to join Granny Squares and other motifs.
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Whipstitch Seam – Place two granny squares side by side, wrong sides up. Be sure to match your stitches as you work along the joining edge. Insert the needle under the top loops of corresponding stitches, pull yarn through and back over, whipping the stitches together. Move to the next stitches and repeat across, joining both edges.
Weave Seam – Place two granny squares side by side, wrong sides up. Insert needle front to back (Square #1 to Square #2), under top loops of corresponding stitches, pull yarn through. In next stitch reverse process. Insert needle from back to front (Square #2 to Square #1), under top loops of corresponding stitches, pull yarn through. In short you are uniting both granny squares with a running sewing stitch. You can also work on the right sides instead if you prefer the running stitches to show.
Video – Simple & Sensational™ – #1 Join Granny Squares with Whipstitch Seam & Weave Seam