They are called Cross Sizes, Neighbour Sizes or Sister Sizes. But what are they actually? If you find a great bra, but it doesn't really fit or the shop doesn't have your size, the best thing to do is to familiarise yourself with Sister Sizes. There you might find what you are looking for.
Sister Sizes are the next size down or up from your size. You can find them in this table. A coloured row (diagonal) shows different sizes that are suitable for the same bust volume. The width of the underband and the cup construction vary.
So if a bra in your size doesn't fit perfectly or isn't available, you can try the Sister Sizes within the same colour group. You can also solve specific fit problems:
1.) Problem: The cup fits, but the underband is too loose.
An underband that is too loose will ride up in the back and may not give your breasts enough support. If the cups hold your breasts nicely but the underband slips, go down a size in the number and up a size in the letter.
Example: 70B becomes 65C
2.) Problem: The cup fits, but the underband is too tight.
If the cup fits nicely but the underband cuts in, go up a size in the number and down a size in the letter.
Example: 70B becomes 75A
3.) Problem: The underband fits, but the cup is too small.
The underband fits comfortably and gives enough support, but the cup cuts in and squeezes or your breasts spill over on the sides? Then stick with the same number, but go up with the letter.
Example: 70B becomes 70C
4.) Problem: The underband fits, but the cup is too big.
The underband is nice and tight and gives enough support, but the cup sticks out or wrinkles a lot? Then stick with the same number, but go down in the letter.
Example: 70B becomes 70A
5.) Other problems
If neither the cup nor the underbust band fit you, the Sister Sizes won't help. Then you should go for a different size altogether.
If the cup and band are too loose, go down a size. So go down a size in the band while keeping the letter the same.
Example: 70B becomes 65B
If the cup and band are too tight, go up a size. So go up one size in the band while keeping the letter the same.
Example: 70B becomes 75B