Letter cards were introduced in Denmark in October 1888.

8 øre letter card upfranked with 3 x 5 øre sent to Brazil. 

5 on 4 øre letter card sent as printed matters to Holland

The perforation is found in four different main types: Perforation 9½, 11 1/4, 10 3/4 and 12.  

See examples from my collection of letter cards.

Before issueing letter cards in 1888, some essays were made.

As written on the edge, the normal use of the letter card will be to tear off the edge of the letter card when opening it. But some cards were opened with the edge being intact. These letter cards are scarce and the value is higher than used letter cards without the edge.