For bras with a frame, the band of the bra is a continuation of the frame that extends to the center back of the bra. If the bra does not have a frame, the band starts right at the outside of the cups. The band of the bra must have stretch for comfort and this stretch is positioned horizontally so the band expands around the body.

Underwires

Now let’s talk underwires. Bra underwires, typically made of metal, are most often located inside of a plush casing that surrounds the cup. Underwires are designed to have some spring in them so that they splay a bit when the bra is worn.

Underwires are the key to support in a bra. To understand how they work, it helps to think of the Golden Gate Bridge. It has two tall towers, each with suspension cables coming down from it that join the deck of the bridge. Those cables look a bit like an underwire, right? Well, they work in a similar way. The downward force from traffic on the deck of the bridge travels up the cables. The cables then transfer that force to the towers and then the towers diffuse the force by transferring it into the earth.

Similarly, in a bra, the downward pressure on the wire is transferred up the sides of the wire and is diffused into the frame and band. Because the stress of support is transferred into the frame, the stronger and wider the frame and band are, the more this pressure can be distributed throughout the rest of the garment.

Both bras and bridges need strong foundations to support the weight they must bear. To provide this foundation, the frame and band must be fitted snugly on the body. If the bra band is too large or too stretchy, the support level is decreased.

Bridges also illustrate why non underwired bras do not provide as much support. Think of a rope bridge: it is anchored on each side, but it sinks down when something heavy is on it. This is because there is no rigid support infrastructure to transfer the downward pressure of the object on the bridge away from the deck. While non-wired bras can be fun to wear for short periods of time, their lack of support makes them generally unsuitable for any kind of application as undergarments. For most women, they are more suited to loungewear or sleepwear.

Straps

You may be surprised that bra straps are not part of the support story, but the primary role of the straps is to keep the bra in the correct vertical position on the body. That is not to say they don’t take on some of the supporting stress, but it is just not their primary job. This is a very good thing since no one wants to feel that kind of pressure weighing on their shoulders.

Bra straps are frequently made completely of elastic but they can also be made of multiple parts with a non-stretch fabric for the front of the bra and an elastic part for the back. They are also sometimes padded for comfort. In terms of strap comfort it is definitely preferable to have at least part of the straps made from elastic. Adjustable straps are also preferable so that as the bra wears out over time the straps can be tightened to maintain the correct garment positioning on the body.

Hardware

With that we come to the bra hardware, the hooks and eyes and the rings and slides. The hooks and eyes securely hold the bra together while the bra is under tension, typically at the center back of the garment. They usually have multiple positions so the bra can be loosened or tightened for the individual figure and can be adjusted over time as the bra elastics wear out. The rings and slides, typically made of metal (though they can also be in plastic) allow the bra straps to be adjusted as described above.

Ultimately, the best way to really learn bra anatomy is to try sewing one for yourself. It is not difficult and it is one of the most satisfying garments you will ever sew. Find yourself a good instructional book, a good pattern, and the right materials (I have suggestions for all three at my site), and have fun!