The DL in DL envelope refers to dimension lengthwise. The DL envelope name comes from Germany, where it was called a DIN lang. DL envelopes are the most commonly used envelopes.
The small and light DL envelope makes postage cost less for individuals and businesses. The ISO-defined size for the DL is 100 millimeters by 210 millimeters. The DL envelope is typically used for sending brochures, flyers, letters, cards and various types of business correspondence.
The DL envelope is one-third of the size of A4 paper, which is the most frequently used paper size. An A4 paper needs to be folded twice in order to properly fit into a DL envelope.