This vowel change has much to do with the overall accent pattern assigned to each word.

I could hear doors slamming.

Patterns of freeze-up and breakup influence the distribution and number of seals, the polar bear's main prey.

The shifting pattern of world trade is made clear in the following table.

A diametrically opposed kinship pattern is to be observed among certain South Sea Island communities.

Accordingly, besides noun declension patterns, there also existed a greater variety of verb conjugation patterns than in Modern English.