The book was written in Hebrew. That language has 22 letters in its alphabet. Each of the verses in chapters 1, 2 and 4 start with the next letter of the alphabet. In Chapter 3 groups of 3 verses also start with the same letters. That is why chapters 1,2 and 4 have 22 verses and chapter 3 has 66 verses. Chapter 5 also has 22 verses, but does not follow the method of using a different letter for each verse. This type of structure is called an acrostic and may have been used as an aid to memory. 