Under the "Advanced Search" option on our Apple Varieties page, you will see 4 different choices for pollination period: Early (A), Early Midseason (B), Late Midseason (C), and Late (D).
These descriptors and letters correspond to the timing of the blossoming out of all of the different apple cultivars that we grow. While a few apple cultivars are considered "self-fertile," most are diploid, meaning that they require another tree of a different cultivar/variety, blooming during the same period, for pollination to be achieved and for fruit to form. Some apples are even triploid or self-sterile, requiring two other cultivars for pollination, and providing virtually no cross-pollination benefit to other trees.
Another factor to consider: some cultivars bloom across more than one period. For example, we often recommend the Grimes Golden apple to help pollinate a diverse orchard, as it blooms across three periods (A, B, and C), and as such is considered a "universal pollinator." Grimes Golden is also thought to be partially self-fertile. Many crab apples provide excellent pollination services as well.