apostrophe用法

For example: cat
Peter's cat / Mary's cat= Peter 的貓 / Mary 的貓
Peter and Mary's cat= Peter 和 Mary 的貓
Peter's and Mary's cat are... (e.g. white)= Peter 和 Mary各自的貓是...(e.g.白色)

This is Peter and Mary's house

Peter and Mary's cat = Peter 和 Mary 的貓 (兩個人共同擁有一隻貓)

Peter's and Mary's cats = Peter 和 Mary 各自擁有自己的貓
cats 是 plural, (因為兩個人合共有兩隻貓)

Without context, it is assumed that Peter has one cat and Mary has one cat.

C of A and B = A and B's C

2013-12-01 09:48:58 補充：
Talking about cat, 意I思A and Godfrey 's common friend is 感恩貓。:)

Peter's dog and Mary's cat are very funny.