Originally Posted by blue eyed pony
And the Trakehner studbook allows crosses to TB's and Arabians (and Anglo's) to make "purebred" Trakehners, for the purpose of refining the breed. In theory you could have a horse that was 7/8 Arabian, 1/8 Trake, but if its parents on both sides were studbook registered/approved it would be registered as a purebred.
This is not QUITE true, at least not in the US. While the horse can be REGISTERED with the ATA, it would not be considered a purebred, nor would it be eligible for inclusion in the OFFICIAL studbook. The percentage of thoroughbred or arab blood cannot be that high for the horse to be considered for Official Stud Book (OSB) approval. For example, if you have a OSB approved "purebred" Trakehner mare whose dam was a Preliminary Stud Book (PSB) approved thoroughbred, you cannot breed the OSB approved mare to an OSB approved thoroughbred or arabian stallion and have that resulting foal be eligible for OSB approval. The stallion cannot have a TB or Arabian parent.
Clear as mud?