Unless you have access to seamless pre-formed foam cups - it is very likely that you will use seams to make the fabric fit over the breast. Don’t be disappointed! The very best European bras, such as Lejaby, and La Perla use seaming, and they never apologize for it. In our school, we have even referred to any seamed bra as having “fine European styling”, but the correct name for it is a “cut-and-sew” cup.
Cut-and-sew cups are more supportive, and have more design options than their seamless counterparts. Why? Most seamless cups are made from foam or stretch polyester fabric pressed into a set shape. If the foam shape is not the same curvature as the breast underneath, the foam cup will collapse. A good example of this is a pointed shape breast trying to fit into a rounded foam cup, especially if the breast is fairly firm. Also, if a seamless cup is made from stretch fabric (no foam), the fabric will continue to stretch under the weight of the breast, so there could be a lot of “bounce” happening with this type of cup.
If you want a fabric made of lace, with the pretty lace edge running along the top edge of the bra, the only real option is cut-and-sew. In that way, the style lines of the cup are designed around the lace, its repeat and the intended shape of the top edge.
Seaming across the cup can follow several orientations; the three most common are the diagonal seam, the horizontal seam and the vertical seam. All seams in a bra cup have one basic rule - they must cross the bust point or darn close to it. The whole purpose of a cup seam is to give the added depth and width the breast needs - to put the seam anywhere but across the bust point is counter-productive. That is not to say, that in the case of a multiple-seam cup, all seams need to cross the bust point; just one of them does. The one that crosses the bust point is the defining cross cup seam.
A horizontal seam, by definition, will start and end in the wire line, so they are fairly easy to spot. Horizontal seams can take any of the variations shown here. They can go straight across, riding on the crest of the bustline, or they can curve downward one end or both. Horizontal cups often incorporate the use of a split lower cup. That’s the designer’s choice. Horizontal seams are the seams of choice for strapless bras or for cups that have a straight top edge - the seam is then placed more or less parallel to the top edge, and it creates a very balanced look to the cup. It is also the best seaming choice for very large cups.
A diagonal seam has its upper end starting somewhere in the armhole curve, anywhere between the strap attachment point and the wire line. Like horizontal seaming, it can be tilted higher or lower within its limited space, depending on the look that is desired. I happen to like the look of a high diagonal seam, as I think it is a more flattering seamline for the wearer. It can also be paired with a split lower cup, although the split is often tilted more toward the side seam, so as to form more of a “T” seam at the cross cup. Compare it to the drawing of the split lower cup on the horizontal cup
The vertical seamline starts anywhere along the top edge of the bra, between the strap attachment point and the wire line near the centre front (often called the neckline edge). A vertical seam starts anywhere along this edge. It may start close to the strap but I have seen it in the middle of the neckline (especially on pre-formed foam cups covered with lace). Having a vertical seam allows the use of lace on the part of the cup closest to the centre front (called the inner cup). This is a totally different look than lace on the upper cup, which would normally be the case in a diagonal or horizontal cup. The seam that runs vertically under the bust gives more support than would normally be present with fabric alone; the stitches can keep the fabric seam from stretching. Vertical seams are also present in cups with a curved seam from the strap. If you look closely, you can see that the seam that runs through the bust point (if carried to the edge) is actually making the bra on the right a vertical seamed cup. It is always the seam that crosses the bust point that determines the name of the seam.