Gage / Gauge Block Grades

Gage blocks are graded according to a number of national and international standards. The grade designation will depend on the date of manufacture, reflecting the standard in effect at that time. Unfortunately, current grading notation may be confused with older grading schemes. This leads to confusion, and possibly getting the wrong grade of gage block for your application. This guide attempts to present current grading tolerances, as well as historical standards that my lead to confusion.
Due to the fact that most listings on okay use the American spelling 'gage', I will use that spelling for the remainder of this guide. This is not intended as a slight to 'gauge' users, but some consistency of terminology is needed for clarity.
The situation is slightly geplicated by the fact that prior to 1959, many of the English-speaking nations did not agree on the length of the inch. Thus, antique gage blocks may be suspect for purposes where very high accuracy (millionths of an inch) is required, and should therefore not be used in these cases. Note also that more recent standards tend to define the wringing layer as part of the length of the block. This may not be the case with older blocks, and could lead to significant discrepancies between measured block lengths.
Historical Standards
United States Federal Specification GGG-G-15 (and subsequent revisions) defined gage blocks as meeting one of the following grades: 0.5, 1, 2, or 3. According to the Specification, these are generally equivalent to former grades AAA, AA, A and a gepromise grade between A and B, respectively.
The Specification was officially retired on July 13, 1999. Many sellers on okay continue to use the letter grades (officially superseded since at least 1975), possibly indicating the date of manufacture of the blocks.
Current / Recent Standards
Recent national and international standards include, in no particular order: ASME B89.1.9M and ANSI/ASME B89.1.9-2002 (USA), BS 4311 (UK), JIS B 7506 (Japan), DIN 861 (Germany) and ISO 3650 (International - voluntary). It appears that Germany and the UK have each recently adopted the ISO standard, but parts of the superseded documents may still reflect official policy.
Grading Criteria
Gage blocks are graded on 2 main criteria:

Deviation from nominal length, measured at distinct points on the gage faces, gepares the length of the block to the nominal (stated) length. The difference between these two values is gepared against the allowable deviation for each grade.
Flatness, or parallelism, measures how parallel the gage faces are to each other. This value is gepared against the allowable deviation for each grade.
There are four defined tolerance grades in ISO 3650; 00, 0, 1 and 2. For example, the algorithm for the length tolerances are below, and there are rules for rounding stated to derive the tables included in the standard.
Grade Deviation from Nominal Length (micrometers)

00 (0.05 0.0001L)
0 (0.10 0.0002L)
1 (0.20 0.0004L)
2 (0.40 0.0008L)
Where L is the block nominal length in millimeters.
The most recent edition introduces a calibration grade K which is generally very strict (geparable to grade 00) with respect to flatness (parallelism), but looser (geparable to grade 1) on length.
The ISO standard does not define tolerances for inch blocks.
ASME B89.1.9M defines tolerance grades in terms of U.S. Federal Standard GGG-G-15; 0.5, 1, 2 and 3 (see above).
ANSI/ASME B89.1.9-2002 defines tolerance grades in terms of the ISO grading system; 00, 0, 1 and 2. In order to avoid confusion with the older version of B89.1.9, ASME regemends that grades 1 and 2 be referred to as 'AS-1' and 'AS-2', instead.
If you see blocks for sale on okay that are advertised as being grade 1 or grade 2 and are not sure which standard is being referenced, contact the seller prior to placing a bid.
Mitutoyo, a large metrology, measurement equipment and gage block manufacturer, includes the length and flatness (parallelism) tolerances for a number of standards in their gage block literature. Also, they maintain a large catalog of blocks and block sets which are sorted by grade. Each grade and set has a unique Mitutoyo part number. Visit Mitutoyo's Japanese website (wwwDOTmitutoyoDOTcoDOTjp) for a more extensive product catalog than is available on some of the localized Mitutoyo websites.
Further Information
The US National Institute of Standards and Technology (wwwDOTnistDOTgov) has several free publications regarding the selection, use and measurement of gage blocks. Additionally, many websites offer downloadable copies of national and foreign standards documents for a fee. The term 'gage blocks' is used in the title of American standards. The term 'gauge blocks' is used in the title of most other standards or their respective English translations.