DL is an internationally agreed code for envelopes of size 110mm x 220mm (4.33ins x 8.67ins). It does not fit into the ISO216 'A' series of sizes, since the aspect ratio is different, although it is so widely used internationally that it is mentioned there.
A sheet of A4 paper, when folded into thirds (two folds, parallel to the short edge) fits snugly into a DL envelope. For decades it was the most popular business envelope size. Most people are able to make the folds accurately enough, by eye, to be sufficiently equally spaced to fit. Our pre-printed stationery template has two feint marks in the margin to help with this.
Since the end of the last century more machinery has entered into envelope distribution. The convenience of folding an A4 sheet only once, and the abilty of machines to better handle the larger size, has meant that the C5 envelope is now the most popuar size for business mailings. Latterly the Post Office and Royal Mail (with, I suspect, international collaboration) permits up to C5 size to travel at the lowest price rate ('Letter') (provided it is not too thick ~ the limit is 5mm, less than a quarter of an inch).
The popularity of envelope sizes works in a beneficial circular fashion with their pricing. C5 and DL envelopes are usually disproportionally cheaper then other sizes. I have not been following price comparisons between them, but I susect that C5 is now cheaper than DL, in spite of material costs being less. Price comparisons are difficult anyway, since many popular items are heavily discounted as 'sale' or 'offer' items.
We have used 'internationally' above. It is worth mentioning that North America has its own range of paper and envelope sizes. I believe that the 'A' series is creeping into availability there, and maybe the 'C' sizes will do so also. Some other countries have their own national long-established sizes, and it is reasonable for them not to widely adopt the ISO range.
