Traditionally, IBs were created by crossing TBs to SDBs.
Most TBs are tetraploid with 48 chromosomes and most SDBs are amphidiploid with 40 chromosomes. Seedlings produced by crossing TBs and SDBs are usually infertile, then, because they have an unbalanced set of chromosomes, but weird things can happen with plant genetics so a few end up being fertile.
There are also other ways to create IBs, including using Iris aphylla which is a naturally 48 chromosome tetraploid species that is about IB-sized. So, you can cross it to TBs and get fertile offspring. If you look deeply enough into Paul Black's fertile IBs, you'll usually find Iris aphylla somewhere.
