Forms of address 

H. S. H. the Reigning Prince 

On envelopes:                      His Serene Highness (H. S. H.)
                                                Prince Hans-Adam II von und zu Liechtenstein
Salutation in letter:              Your (Serene) Highness
Oral address:                         Your (Serene) Highness
Complimentary close:         Please accept, Your Highness, the assurance of my highest consideration.
                                            
H. S. H. the Hereditary Prince

On envelopes:                       His Serene Highness (H. S. H.)
                                                 Hereditary Prince Alois von und zu Liechtenstein
Salutation in letter:              Your (Serene) Highness
Oral address:                         Your (Serene) Highness
Complimentary close:         Please accept, Your Highness, the assurance of my highest consideration.

H. R. H. the Hereditary Princess

On envelopes:                       Her Royal Highness (H. R. H.)
                                                 Hereditary Princess Sophie von und zu Liechtenstein
Salutation in letter:               Your (Royal) Highness
Oral address:                          Your (Royal) Highness
Complimentary close:          Please accept, Your Highness, the assurance of my highest consideration.

H. S. H. the Hereditary Prince and H. R. H. the Hereditary Princess together

On envelopes:                      His Serene Highness (H. S. H.) Hereditary Prince Alois and 
                                                Her Royal Highness (H. R. H.) Hereditary Princess Sophie von und zu Liechtenstein
Salutation in letter:              Your (Serene) Highness, Your (Royal) Highness
Oral address:                         Your (Serene) Highness, Your (Royal) Highness
Complimentary close:         Please accept, Your Highnesses, the assurance of my highest consideration.