pres. prepos. 1
If the contraction is followed by a vowel sound, du and de la both become de l' and au and à la both become à l'. This change occurs for euphony only; the nouns do not change genders because of it.
This language is not understandable!
Basically, if the noun starts with a vowel, and you would have du or de la before it, it turns into de l' or a l'. The gender of the noun doesn't change, only the way you say it does. Hope this makes sense,