Introduction to Kilayim
The word kilayim means mixture, either a mixture of seeds, plants, cross-bred animals or even cross-yoked animals. The Torah twice prohibits kilayim.
You shall not let your cattle mate with a different kind (kilayim); you shall not sow your field with two kinds of seed (kilayim); you shall not put on cloth from a mixture of two kinds of material (kilayim shatnez).
You shall not sow your vineyard with a second kind of seed (kilayim), else the crop from the seed you have sown and the yield of the vineyard may not be used. You shall not plow with an ox and a donkey together. You shall not wear cloth combining wool and linen (shatnez).
The rabbis interpreted You shall not sow your field with two kinds of seed to mean that planting two different kinds of seed in a field was forbidden and that grafting one type of plant onto another was also forbidden. You shall not sow your vineyard with a second kind of seed meant that one could not plant either grains or vegetables in a vineyard.
Planting two different kinds of seeds in one field is only prohibited if the seeds are mixed up together. If they are planted separately and there is a divider, one can plant two different kinds of seed in one field.
In total there are four types of kilayim: 1) kilayim of the vineyard; 2) kilayim of mixed seeds in a fieldthis prohibition includes a prohibition of grafting together different trees; 3) cross-breeding animals; 4) yoking two different types of animals together. Our tractate will discuss all four different types.
Good luck learning Tractate Kilayim!