Regarding Shabbat, we learned: "The primary prohibited acts (avot melakhah) are forty less one" (Mishnah Shabbat 7:2). "Primary prohibited acts" implies that there must be secondary prohibited acts (toldot). The secondary acts are the same as [the primary ones]; there is no difference between a primary and a secondary [prohibited act] . . .
We have seen before that Mishnah Shabbat 7:2 lists the thirty-nine types of labor (melakhah) prohibited on Shabbat. Till now we have ignored the fact that the Mishnah defines these labors as the "primary prohibited acts (avot melakhah)." There are, in fact, many more than thirty-nine specific Torah- prohibited acts on Shabbat. For instance, the Mishnah prohibits sowing seed, but never mentions the planting of a sapling tree, though the result of both acts is substantially the same: a plant flourishing in a previously empty location due to human intervention. Does the Mishnah's silence on saplings imply that we may do such planting on Shabbat? Obviously not. The primary prohibited act is, as defined by the Mishnah, sowing seed. This act is called an av melakhah (primary category of prohibited labor). Similar acts derived from this primary category are termed toldot (singular, toldah; secondary or derived prohibited acts). Planting a sapling on Shabbat is prohibited because it is a secondary prohibited act (toldah) derived from the primary category (av melakhah) of sowing seed, which we do find in the Mishnah's list.
One can see the literary necessity of using major categories to list Shabbat prohibitions in a document as brief as the Mishnah. There simply is not enough space to list every possible violation of Shabbat. The system of avot and toldot allowed the author of Mishnah Shabbat 7:2 to restrict the number of listed categories to thirty-nine acts, and let later generations derive the remainder. Indeed, the Talmud Yerushalmi tells us of two Sages who spent years working on Mishnah Shabbat 7:2 until they had derived thirty-nine toldot from each of the thirty-nine avot melakhah listed. This system of derivations may have its roots in the Mishnah's terseness, but it has allowed a dynamism and innovative spirit that has kept questions about Shabbat vibrant in times of changing technology. May one use a camera, or is it a toldah of writing? May one turn on the ignition of an automobile, or is it a toldah of lighting fire? The system explained here has ensured that we have a religious language to discuss these new questions.