Maseches Kilayim, Chapter 9, Mishnah 1. This chapter deals with the laws of shaatnez - the prohibition against wearing a garment that contains a mixture of wool and linen together. The Mishnah opens by defining the scope of the prohibition, and from that definition further laws unfold that apply to these two materials alone.
"Ein assur mishum kilayim ela tzemer u'fishtim":
Nothing is forbidden under the law of kilayim - meaning here shaatnez - except wool and linen. These are the only two materials subject to the laws of kilayim in garments.
Alongside this, the Mishnah lists two additional laws in which wool and linen specifically are singled out:
"Ein metamei bi'nega'im ela tzemer u'fishtim" - the only types of garments that become tamei through the plagues of tzaraas are those made of wool or linen.
"V'ein ha'kohanim lovshim l'shameish b'Veis HaMikdash ela tzemer u'fishtim" - the kohanim do not wear any other type of garment for their service in the Beis HaMikdash, only garments made of wool or linen.
Which wool is forbidden:
Not every type of wool is forbidden in shaatnez. The forbidden wool is the wool of sheep alone; the wool of deer or of camels is not subject to the laws of kilayim.
"Tzemer gemalim v'tzemer rechelim she'terafan zeh ba'zeh":
When one takes camel's wool and sheep's wool and blends them together into a single mixture, the halachah is determined by the majority:
"Im rov min ha'gemalim - mutar" - the sheep's wool is nullified in the majority, and it is permitted to combine the mixture with linen in a garment.
"V'im rov min ha'rechelim - assur" - the mixture is considered sheep's wool, and it is forbidden to combine it with linen.
"Mechtzah l'mechtzah - assur" - when the amounts are equal, the wool is not nullified, and this too is forbidden.
"V'chein ha'pishtan v'ha'kanbos":
That very same rule applies to a mixture of flax and hemp (canvas), which is not subject to the laws of shaatnez: if the majority is flax - it is forbidden; if the majority is hemp - it is permitted; and half and half - it is forbidden.
In summary: in this Mishnah we learned that the prohibition of kilayim in garments - shaatnez - applies only to wool and linen, and likewise that only wool and linen garments become tamei through the plagues of tzaraas and only they are worn by the kohanim for their service in the Mikdash. We also learned that the wool referred to is specifically sheep's wool, and therefore in a mixture of camel's wool and sheep's wool, as well as in a mixture of flax and hemp, everything depends on the majority - and half and half is forbidden.
As the chapter continues, we will go further and deeper into the details of the laws of shaatnez.