Window is divided "win/dow ". The second syllable has two vowels together. It follows rule # 8.
When a syllable has 2 vowels together, the first vowel is usually long and the second is silent. Examples: "pain, eat, boat, res/cue, say, grow". The letters "ow" can also signal a diphthong which sounds like "ou", but in this case, we follow the rule.