A record according to this standard consists of 3 parts - the leader, the directory, and the variable fields - and is terminated with ASCII code #29 (Ctrl-]).
The leader is a fixed length of 24 characters (positions 0-23), assigned as follows:
|6||type of record|
|11||identifier length* (including delimiter)|
|12-16||base address of data* (=leader length+directory length)|
|20||length of length-of-field portion*|
|21||length of starting-character-position portion*|
|22||[length of implementation-defined portion*]|
The directory consists of a variable number of entries and is terminated with ASCII code #30. Each entry consists of the following portions:
|tag||3 characters, alphanumeric||starting with "00" for a control field, with "001" reserved for a control number field and "002" for a subrecord field|
|length of field||0-9 characters, decimal||with the length including the field terminator, and with zero indicating the maximum length with the next entry a continuation field|
|starting character position||0-9 characters, decimal||relative to the base address of data|
|[implementation defined]||0-9 characters|
In the variable field part, control fields are again ordered by tag and followed by other fields in any order. Generally, there may be more than one field with a given tag, but there is always exactly one control-number field. Each non-control ("data") field starts with a fixed-length indicator. A data element (subfield) within a non-control field may be identified by being preceded by a delimiter (ASCII code #31) plus an identifier. Control fields do not contain indicators, delimiters, or data element identifiers.
Before harmonization in 1997, a slightly different bibliographic format (CanMARC) was used in Canada:
The file marcdemo.mrc contains a single sample CanMARC record. Since it contains control characters, it cannot be fully displayed directly, but, when divided into 12-character blocks, it starts with the following leader and directory:
00504nam 22 001810a 4500 001001300000 008004200013 016001700055 020001000072 040002400082 055001100106 082001200117 100003000129 245005300159 260004200212 300003300254 650001600287 650001900303 ...
The first 5 characters ("00504") in the sample record indicate, of course, that it is 504 characters long, which, in this case, is also the length of the file.
In the leader of a MARC record, the record status character is used to indicate whether the record is new or deleted; the type of record character indicates such types as printed language or printed music materials; the bibliographic level character, whether the item is a part of a monograph, a serial, or the like. Character position 8 is used for type of control (archival or not). The next 4 characters ("nam ") in the sample record can thus be interpreted as meaning "new, printed language, monograph, not archival". The MARC indicator and identifier lengths are always 2. Thus the first block of 12 characters in the sample record ends with "22".
The base address of data in the sample record can be seen to be 181 ("00181"). Character position 17 is used for encoding level (full, partial, etc.); position 18, for descriptive cataloging form (ISBD, AACR2, etc.); and position 19, for linked record requirement (yes or no). The blank at position 19, following the "0a", in the sample record indicates that no linked record is required to supply additional information about the cataloged item.
The MARC entry map is always "4500"; that is, in each entry the length-of-field portion is 4 characters, the starting-character-position portion, 5 characters; and there is no implementation-defined portion.
The directory entries can be analyzed as follows:
|Tag||Length of field||Starting character position|
If we take the 30 characters starting at position 181+129=310, we shall have the contents of the 100 field, the main entry. Representing ASCII codes #30 and #31 with ¶ and § respectively, this appears as
1 §aPilcher, F. E. V.,§d1912-¶
If linked records are required for a MARC record, they can be identified in a subfield of the appropriate linking entry field. The format commonly used is the control number of the linked item preceded, in parentheses, by the MARC code for the agency to which the control number applies (e.g, "(CaOONL)").