Corner card
The term corner card means the wording, sometimes with a pictorial feature, in the upper left hand corner of a postal stationery envelope or an envelope designed to have regular adhesive stamps affixed to it. It is there for the purpose of stating the sender's return address to facilitate the return of undeliverable mail. Classification
There are four general classifications of corner cards:  

 Partial request corner cards are a mere outline, letting the sender fill in the number of days for an undelivered envelope to remain before return plus the sender's entire name and address, e.g. "After __ days, return to ...";
 General request corner cards have the printed name of the state or possession, e.g. "Philippine Islands";
 Post Office request corner cards have the printed specific name of the post office, e.g. "Cristobal, Canal Zone";
 Special request corner cards have the full name and address of the sender. These were used by commercial concerns with a large volume of correspondence. They would normally be ordered in multiples of 500. See also
 Postcard
 Trade card
 Trading cards

References

External links
Postage Stamps

Postcards
Postal stationery
Philatelic terminology