Like most fruits and nuts, hazelnuts come in many varieties. Two varieties are needed for cross pollination and nut production and not all varieties are compatible with each other.
The female, nut bearing, flower must also be receptive at the same time that pollen is release by the catkin (male flower). Since pollen release and receptivity occur at different times for different varieties, genetic compatiblity alone isn't enough to ensure nut production. That's why commercial orchards use two or three compatible pollinizer varieties for each select nut variety.
It sounds a bit complicated, because it is! But we have taken the headaches out of variety selection on our recommended mixes page.
All hazelnut trees can have nuts and all produce pollen. For example, Yamhill and Sacajawea are compatible pollinizers for each other.
In the past, varieties selected primarily as pollinizers tended to have low productivity and poor nut quality but that's all changed. And more is being discovered all the time about the effect of pollen on nut yields. As a result farmers are opting for more diversity and higher proportions of pollinizers.
Some may even plant 1/3 each of three intercompatible varieties.