Stam or stam lines refers to the recorded lines of significant breeding mares in the breed. It is essentially the motherline, that is, the bottom-most line of names on a pedigree (Dam, Grand-dam, Great-grand-dam..etc). While many are familiar with the more common practice of tracing lineage via the sire, the dam plays an equal role in the production of quality offspring, and there are certain lines of mares which are noted for their history of producing approved stallions.
For a good listing of the Stam Lines, see Signature Friesians’ page on Mother-lines, or check out the Facebook page KFPS Stamline Offspring.
If you can find it, the best resource which documents the stam lines, their founders their unique characteristics is “Friese Merriestammen” (also known as the Mare Books), initially published in 1989 by R. Geurts in two volumes, and last updated in 1997. The books can be hard to come by, and aren’t cheap, but offer a great resource to interested breeders, and each book contains the full text in three languages: Dutch, English and German.