What is a quarterbred? The obvious answer is "something that's one-quarter aril" – and that's good enough for most purposes. It does, however, dodge the question of how aril content is determined in the various classification systems and that has changed over time. To get the most out of the various Checklists and articles written in various periods, it helps to understand how the term has evolved.

It was originally borrowed from the "blood" system used for people and animals, meaning simply one aril grandparent and three non-aril grandparents. It could also mean two aril great-grandparents and six non-aril great-grandparents, of course, if two quarterbreds were crossed successfully – but that was extremely rare.

These were just fertile enough, however, that they were used in a number of complex lines. Different types of seedlings started to proliferate, such as "1/8-breds"and "3/8-breds". The Aril Society adopted the quantum system and developed a special set of classification codes to eliminate the need to specify these fractions. Varieties that were at least one quarter but less than half regelia were designated as RB-; those that were at least one quarter but less than half oncocyclus were designated as OB-; and those that were at least one quarter but less than half aril, with both onco and regelia ancestors were designated as OGB-.

Although most of the varieties classified as RB-, OB- or OGB- were also quarterbreds in the former sense, the term "quarterbred" went out of fashion for a while. As hybridizing continued, however, lines became increasingly complex and eventually some arilbreds were developed that were fully fertile even though their total aril ancestry was less than half. This system of classification based on ancestry rather than the characteristics of the individual plant therefore led to the inclusion of some fully fertile varieties in the OGB- group, which in turn led to another change in classification systems.

The Aril Society now defines a quarterbred as having at least one set of aril chromosomes and more sets of non-aril chromosomes than aril. In practice, that means the term "quarterbred" includes both tetraploids (with one set of aril chromosomes and three sets of non-aril chromosomes) and triploids (with one set of aril chromosomes and two sets of non-aril chromosomes) -- and it does not matter which route led to the birth of that seedling.  

As a result, the new definition is relatively close to the original one and convenient for most purposes. The hybridizer, however, needs to understand both the similarities and differences in order to work with this group effectively.

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