Blood Royal is a 350 page book tracing British Royal lineages from the Roman Emperors and native British sovereigns, through all royal houses to the present-day Windsors.

STRICTLY speaking, 'blood royal' is a pseudo-scientific concept. Did the late Queen Mother become any less royal when she had a blood transfusion in the summer of 2001? Did the donor - and we don't even know the person's sex, let alone whether (s)he was aristocrat or commoner – acquire vicarious royalty by providing the gore? Of course not. What is passed down from ancestor to descendant is a bundle of genes, predispositions to certain illnesses, facial and bodily features, some degree of intelligence, and most traits of character, but not actually blood, not even blood groups - particularly if one has a transfusion years after bearing children, as the Queen Mother did. Still, as a phrase in popular use 'blood royal' is immediately intelligible.

Descent from sovereigns, most people would say. Preferably dead ones, they might add after some thought. After all, descendants of a living one are so few they comfortably comprise that much-exposed kinship group we call 'The Royal Family'. Sovereign, by the way, is to be understood technically, as a supreme ruler. Thus Oliver Cromwell is included in this book, plus his descendants, including the present Duchess of Kent. Also such usurping oddities as Carausius, Count of the Saxon Shore. Ethnic minorities too: one sovereign who numbered Britain among his realms may have been black, Septimius Severus. And women. Some of the most forceful sovereigns in Britain have been queens - Boadicea (or Boudica), Cartimandua. Not to mention immigrants, many sovereigns being imports, ambitious princelings from foreign parts scrambling aboard the British throne as a good career move, real promotion. William I and III vaulted from ducal or princely rank to independent kingship just by crossing the Channel. George I was only an elector in Germany. Despite its prestige within the Holy Roman Empire, this position for practical purposes usually involved being one of seven (later eight, later still nine) stooges whose vote on imperial polling day was a foregone conclusion since the winner was invariably a Habsburg, just as the Communist Party candidate always triumphed in pre-1989 Russian elections.

So far so good. But how close must this descent from sovereigns be to qualify? How 'born to the purple', as it were, must those with sovereign ancestors be for their blood to merit the magic qualifier 'royal'? An altogether trickier question. Take Edward ill, a splendidly fertile king who, moreover, begot his children in wedlock, so that their family details are well tabulated. The Queen is of the twenty-first generation in descent from him, but many people currently of child-bearing age would be more like twenty-three generations. As ancestor he is shared by huge numbers of Britons and Americans today.

How huge? Folk wisdom talks of a million to two million, perhaps even three. The Marquis de Ruvigny (1868-.1921), the Franco-Scottish genealogist who is the inspiration for this book, a century ago compiled a series of five books called The Blood Royal on living descendants of the late medieval kings of England. He came up with about 50,000 for those of Edward III, pinpointing every individual in the lines of descent he was able to follow up. Multiply by 56 million divided by 45 million to take in the general population increase since and we would have about 62,000. But since a disproportionate percentage of Ruvigny's people were upper-class and his book appeared on the eve of World War I, it might be wise to shrink that figure to accommodate the high attrition rate of public school men in the Flanders mud of 1914-18, plus the 'flu epidemic immediately afterwards which culled both sexes and all classes. Say from 62,000 down to 60,000. On the other hand Ruvigny worked mostly from published sources such as peerage reference books. This is not to disparage his very impressive work. But inevitably he overlooked more plebeian descents, if only because they hadn't been well documented.

Part 2 of 7.