What is a slot seam?
A slot seam is a semi-decorative seam construction featuring two fabric folds that meet at the seamline and a strip of either contrast or identical fabric underneath the two folds.
When the sam's abutted folded edges are lifted/separated, the separate fabric layer underneath is exposed in a slot-like construction. In a slot seam application, topstitching is used not only as a means to functionally construct the seam but to also contribute to the decorative nature of the seam folds.
The most intriguing aspect of a slot seam is that it looks a lot more complicated to sew than it really is. Using a temporary machine basting technique to sew the seam in a conventional fashion, allows you to keep the seam edges perfectly aligned in the process of aligning and topstitching the underlaying fabric strip. Once the fabric strip is connected by topstitching, the machine basting is simply removed to expose the finished slot structure on the seam's right side.