Design Context: OUGD504 - Design For Print: Format

ISO216 - A, B
The A series paper sizes are defined in ISO 216 by the following requirements:

The length divided by the width is 1.4142
The A0 size has an area of 1 square metre.
Each subsequent size A(n) is defined as A(n-1) cut in half parallel to its shorter sides.
The standard length and width of each size is rounded to the nearest millimetre.

The B series paper sizes were created in order to provide paper sizes that weren't covered by the A series. As with the A series paper sizes, the standard lengths and widths are rounded to the nearest millimetre.

ISO217 - RA, SRA
The RA and SRA paper formats are defined by ISO 217 "Paper - Untrimmed Sizes" and cover untrimmed raw paper for commercial printing.
The RA and SRA sizes are slightly larger than the corresponding A series sizes to allow for bleed on printed material that will be later trimmed to size, often for bound publication.
RA stands for "raw format A" and is conceptually defined as being 105% of the A series size, thus as A0 has an area of 1 square metre RA0 has an area of 1.05 square metres.
SRA stands for "supplementary raw format A" and is conceptually defined as being 115% of the A series size, so a sheet of SRA0 paper has an area of 1.15 square metres.
In reality the sizes for RA0, RA1, RA2, SRA0, SRA1 and SRA2 are rounded to the nearest centimetre and sizes for RA3, RA4, SRA3 and SRA4 are rounded to the nearest half centimetre.

ISO269 - C
C envelopes sizes are defined as the geometric mean of the A and B sizes with the same number i.e. C4 dimensions are the geometric mean of A4 and B4. This produces a size between the two that makes an envelope that will neatly hold the A series paper of the same size, thus a C4 envelope is perfect for an A4 sheet of paper unfolded.

ANSI/ASME Y14.1
North America, including the US, Canada and parts of Mexico, is the only area of the first world that doesn't use the ISO 216 standard paper sizes, instead they use Letter, Legal, Executive and Ledger/Tabloid paper sizes and those that have been formalised in ANSI Y14.1M - Metric Drawing Sheet Size & Format.
ANSI defined a regular series of paper sizes based around the Letter (8.5" x 11") format, with this becoming the A sizes and larger sizes being B,C,D & E. Surprisingly these ANSI standard sizes were defined in 1995 well after the ISO standard sizes.