Let's start with a big picture and work our way down to the answer. The International Tolerance Grade, also known as IT Grade, identifies what tolerances a specific process can produce for a given dimension defined in ISO 286.
It establishes the magnitude of the tolerance zone or allowed amount of part size variation for internal and external dimensions.
There are totally 18 grades of tolerance, the smaller the International Tolerance Grade number, the smaller the allowable tolerance zone, the higher the precision level.
ISO and ANSI both group fits into three categories: clearance, location or transition, and interference. Within each category are several codes to define the size limits of the hole or shaft - the combination of which determines the type of fit.2
Upper vs Lower Case
The tolerance symbol is established by combining the IT grade number and position letter for tolerance. The first thing to understand is the difference between a capitol "H" and a lower case "h". The upper case "H" is used to designate bore tolerance while the lower case "h" is used to designate shaft tolerances. Tolerances with lower case “h” are (minus) negative. Said another way, the capital letter "H" is for the hole, and lower case letter "h" is for the for shaft.
The number following the letter "h" denotes the International Tolerance (IT) grade. So, a tolerance with the number 6 will have a larger tolerance range than the number 5 but a smaller tolerance range than the number 7. This range is based on the size of the shank.
A hole that has a 0.030” diameter will have an h6 tolerance of (+0.0000,-0.0002), while a 1.00” hole with have an h6 tolerance band of (+0.0000,-0.0005).
2. Shaft tolerances are now covered in the new ANSI B32.100- 2005 standard