Different brief sewn patterns

[Torchwatch]

Will try to answer you question/points, but it may get complex.

When sewing a seam you have to stretch the fabric while sewing it or you would get a "bow tie" effect. I have never wanted this but someone might want it for something.

When sewing a seam along a convex curve a pouch is formed, joining enough convex curve seams together would give you a ball.
A swimsuit without a pouch (used by Speedo etc as a racing brief) is more properly called a smooth front bikini. On the front of a suit the sewn convex seam starting at the lowest point of the suit need not go all the way to the waist line to form a front pouch and achieving a posing suit. A seam going all the way up can be called a contour suit.

Sewing a concave curve seam produces a fork, if you were making a full body lycra suit from a front and back panel you may well sew from left ankle to right ankle via the crotch, the crotch in this case being a concave curve seam and defining the separate legs of the suit. This might be relevant in square cut suits.

When sewing a straight seam the 2 pieces of fabric become one, but they cannot stretch along the seam any more then the fabric was stretched at the time of sewing. Ageing Lycra loses it skin tight qualities and begins to sag, the seat of your swim suit may sag down towards you knees, this looks bad especially on a branded suit so the manufacturer may choose to insert a rear seam to support the seat of the suit. This was essential in Paper Lycra suits which had a very short use life. if the ream seam is slightly less stretchy than the seat of the suit it will be pulled up into ones ass crack a bit making the suit more skin tight and less tunnel forming.

To add a colour band to a suit a section of fabric may be cut out and substituted for the original using a fully stretched straight seam. A convex fabric sewn to an equal and opposite concave fabric are equivalent to a straight seam. If the manufacturer is making a batch of blue suits with white sides he would use straight seams to sew in the side sections. He might use the same sewing pattern to finish the order with plain blue suits but cutting and sewing blue sides into blue suits. (This is my guess)

If you can just about sew a little and would like to see how a suit fits together (I made my first suit with a cut up pvc shopping bag and sticky tape) then private message me with an email address and I could email you the Stitchboy Men's Swimwear Sewing Patterns that were once on the internet. Give it a go ......