One the best identifying features the thickness the card

3.1.1 Card formats

Small cards with the typical smart card dimensions of 85.6 mm by 54 mm have been in use for a very long time. Almost all smart cards are produced in this format, which is certainly the most familiar. It is designated ID-1, and its size is specified in the ISO 7810 standard. This standard originated in 1985 and thus has nothing to do with smart cards as we know them today, as can easily be seen from the abbreviation ‘ID’, which stands for ‘identification’. This standard simply describes an embossed plastic card with a magnetic stripe that is intended to be used for the identification of a person. When it was written, no one had thought of putting a chip into the card. The presence of a chip and location of its contacts on the card were only defined several years later in other standards.

The formats are defined in the relevant standards in a way that simplifies measuring the card dimensions. For instance, the height and width of an ID-1 card must be such that it fits between two concentric rectangles as shown in Figure 3.1 (ignoring the rounded corners), which have the following dimensions:

r external rectangle: width: 85.72 mm (3.375 inches) height: 54.03 mm (2.127 inches)