A sheet of B4 paper with a brown C4 envelope laid on top
Talking numbers and ISO codes is no way to really get to grips with the size of papers. The notes ~ above ~ are there just for the record. Certainly do not make any attempt to learn them. When you need to know them exactly ~ for some reason ~ then they are there ~ and in several other places on this site. If you use them a lot then you will learn them without any effort. Normally you will just associate size on sight with the code numer. Bookcrafters try to avoid using rulers as measuring aids ~ it is easier just to 'copy' the measurement that is relevant [110138].
If you are new to the system I would suggest that you familiarize yourself with just one size at first. A4 is the size most commonly met with. It is the size of most office papers in use today. Every official document or letter that comes in the post is likely to be printed on A4 sheets of paper. Most home desktop printers will take, and mainly use, A4 paper.
Then, for stage 2. remember that A5 is an A4 sheet folded once. A6 is a quarter of an A4, or an A5 sheet folded once. Then, following the system to other way, an A3 sheet is equivalent to two A4 sheets laid down side by side. If you use that vocabulary for a while it will become familar. Time to launch into the B series.
The B series of papers works in much the same way as the A series. Number-for-number the B series is larger. If you want to cover an A4 card by pasting some decorative paper all over it, and over the edges as well, you could use a piece of B4 paper, which would allow a good sized 'hem' to act as a wrap-around. In some ways the B series is easier to understand and remember. I have an introductory note on that [117507].
As suggested ~ in the pictures above ~ there is a fairly strong tradition that decorative papers are made in sizes that follow the B series of sizes. If you buy a pack of special gift wrap it will probably be one sheet of B2 paper, folded twice ~ French Fold [126485]~ to make a package of size B4.
It is worth noting that an unfolded sheet of card, of exact size B4 may travel by Royal Mail as a Large Letter. If the sheet is wrapped with some protective wrapping paper it will be too large ~ and must be paid for as a Small Parcel ~ which will cost about three times the price.
