DL Size or 1/3 A4
One size causes a lot of confusion, and unfortunately, Google doesn't help!
When you fold a piece of A4 paper into 3, you end up with a trendy paper size - with no agreed name!
In the age of the Compliments Slip 'one-third A4' written 1/3 A4 was an overall office paper size. When letters were folded into three to fit into a window envelope, people knew to order DL envelopes. Today DL size has been 'adopted' as the name for this size (although the paper size is slightly smaller than the envelope so that they fit).