Introduces 2 spellings for a vowel sound.

Book 1: ai, ay
Book 2: ee, ea
Book 3: oa, ow
Book 4: ur, ir
Book 5: e, ea
Book 6: ow, ou
Book 7: oo, ue
Book 8: igh, i
Book 9: 00, oul
Book 10: or, aw
Book 11: oy, oi
Book 12: ar, a
Book 13: air, are
Book 14: ear, eer