Fit
Sister sizing
Cup letters are relative, not absolute. Change the band and the same letter changes volume, which is why moving sideways through the size chart is a real technique.
A cup letter describes the difference between the underbust and the overbust measurement, so it is always measured against the band it sits on. A D cup on a 30 band holds noticeably less than a D cup on a 38 band, because in both cases the letter means four inches of difference and four inches added to a bigger circumference produces a bigger volume.
Sister sizes are the pairs that hold approximately the same cup volume on different bands. Go down one band size and up one cup letter, or up one band size and down one cup letter, and the amount of space in the cup stays roughly constant while the tightness of the band changes.
The pattern
| Cup volume | Tighter band | Measured size | Looser band |
|---|---|---|---|
| Small | 30D | 32C | 34B |
| 30DD | 32D | 34C | |
| 30E | 32DD | 34D | |
| Medium | 32E | 34DD | 36D |
| 32F | 34E | 36DD | |
| 34F | 36E | 38DD | |
| Larger | 34FF | 36F | 38E |
| 36G | 38FF | 40F |
Read any row across and the three sizes hold close to the same volume. They are not identical: the cup on a larger band is cut slightly wider and shallower, and the wire is a different width. Close enough to be useful, far enough apart to be worth testing on the body rather than assuming.
When to use it
- The band is right and the cup is not. Do not sister size. Keep the band and change the cup letter only.
- The cup is right and the band rides up. Go down a band and up a cup: 34D becomes 32DD.
- The cup is right and the band digs in or the wires sit on tissue at the side. Go up a band and down a cup: 34D becomes 36C.
- A style is not stocked in the measured size. A sister size is the sensible substitute to try, though the fit will not be identical.
- A brand runs consistently tight or loose in the band. Sister sizing is how to compensate without abandoning a style that otherwise works.
Sister sizing is a way of moving round a size chart, not a way of avoiding the size that actually fits.
Where it breaks down
The technique assumes the cup shape scales cleanly, and it does not. As bands get larger, wires get wider. A 38C wire is wider than a 34E wire even though the cups hold similar volume, so on a narrow root the larger band will place the wire out on the ribs where it presses, and on a wide root the smaller band will cut across tissue at the side.
Depth changes too. Two cups of the same volume can be built as a deep, narrow shape or a shallow, wide one. Projected tissue needs depth; wide, shallow tissue needs the opposite. This is why one label can fit perfectly and another in the same printed size can be unwearable. It is also why the useful question in a fitting room is never which size, but which shape.
Using it in practice
- Start from the measured size, not from the size that has been worn for years.
- Fix the band first. Nothing about the cup can be judged on a band that is moving.
- Once the band is right, adjust the cup letter alone until the top edge lies flat and nothing spills at the side or under the arm.
- Only then, if a specific garment is unavailable in that size, step to the sister size and re run the same checks.
Two sizes will often both feel acceptable in a shop and only one will still be acceptable at the end of a long day. Where there is doubt, the firmer band is usually the one that ages better, because elastic only ever loosens. Band and cup explains why.
Is a sister size the same garment in a different label?
No. It is a different cut with a similar internal volume. The band length, wire width and cup depth all change, which is why it has to be tried rather than assumed.
Can sister sizing fix a band that rides up?
It can, and that is its best use. Dropping a band size and adding a cup letter keeps the volume and tightens the loop that carries the load.