Page "Tar (file format)" Paragraph 2

A tar archive consists of a series of file objects.

Each file object includes any file data, and is preceded by a 512-byte header record.

The file data is written unaltered except that its length is rounded up to a multiple of 512 bytes and the extra space is filled with zero bytes.

The end of an archive is marked by at least two consecutive zero-filled records.

( The origin of tar's record size appears to be the 512-byte disk sectors used in the Version 7 Unix file system.