Most breeders will discard potential matings that produce inbreedings within the first two generations. Sire-line information will tell you how successful the sire of the stallion and some of his paternal half siblings have been. Information on other sons of the sire of each stallion is detailed in the new Active Sons Index. It is true that some stallions are better sires of successful sires than others, but, in fact, each stallion is an individual case. The female line gives you an indication of the general quality of that half of the pedigree.
Successful sires are generally from better female families than unsuccessful ones.
The Directory also lists information on active sires from the female family. Successful sires are more likely to come from families of other successful sires, just like they are more likely to come from established successful lines. For devotees of Dosage, Roman dosage profiles and statistics are included. If you have the profile of your mare, you must delete fifth generation sires from the hypothetical pedigree and add up the new dosage for each prospective foal. As any gambler will tell you, it is always safer to play established form than to bet on a horse to do something he has never done.