Three keywords are reserved by this proposal, CHECKSUM, DATASUM, and CHECKVER. Normally both the CHECKSUM and DATASUM keywords will be present in a header if either keyword is present. This is not required however, and software should be prepared to handle each keyword separately. Each of the three keywords should be written using the rules of FITS fixed format character strings. An exception to this requirement is that the entire 16 characters of the CHECKSUM keyword value and also the entire length of the DATASUM keyword value are significant since all of the characters are needed to encode the checksum or format the 32-bit unsigned integer value of the datasum. The checksum keywords are constrained to fixed format since the ASCII encoding depends on the byte offset within each header card image (see §3.2).