2 NIST Policy on the Use of the SI
In accordance with various Federal Acts, the Code of Federal Regulations, and
Executive Order 12770 (see Preface), it is NIST
policy that the SI shall be used in all NIST publications. (Footnote 1) When the field of application or the special
needs of users of NIST publications require the use of other units, the values
of quantities shall first be expressed in acceptable units, where it is to be
understood that acceptable units include the units of the SI and those units
recognized for use with the SI; the corresponding values expressed in the other
units shall then follow in parentheses. (For precise definitions of the terms
"units of the SI" and "acceptable units" as used in this Guide, see
Sec. 5.4). Exceptions to this policy require the
prior approval of the NIST Director. The following three sections -
2.1 Essential data, 2.1.1 Tables and
graphs, and 2.2 Descriptive information - elaborate upon
this policy.
2.1 Essential data
Essential data express or interpret quantitative results. All such data shall
be given in acceptable units. In those cases where
- the sole use of acceptable units would compromise good communication, or
- units other than acceptable units have been specified as a contractual
requirement,
values of quantities shall be given in acceptable units followed, in
parentheses, by the values of the same quantities given in the other units.
Exceptions may sometimes be necessary for commercial devices, technical
standards, or quantities having special legal significance; examples include
commercial weights and measures devices and the related laws and regulations.
However, even in such cases, values of quantities expressed in acceptable units
should be used when possible with the same values expressed in other units
following in parentheses.
2.1.1 Tables and graphs
In tables, values of quantities expressed in acceptable units and the
corresponding values expressed in other units may be shown in parallel columns,
with the acceptable-unit column preceding the other-unit column. In graphs,
axes labeled in other units shall be given secondary status. This may
preferably be done by placing scale marks on and labeling the left-hand
ordinate and bottom abscissa in acceptable units, and placing scale marks on
and labeling the right-hand ordinate and top abscissa in other units.
Alternatively, lighter-weight scale marks and smaller type may be employed to
indicate other units using the same ordinate and abscissa as is used for the
acceptable units.
2.2 Descriptive information
Descriptive information characterizes arrangements, environments, the
generalized dimensions of objects, apparatus, or materials, and other
attributes that do not enter directly into calculations or results. When
necessary for effective communication, such information may be expressed using
customary terms that are widely used and recognized. Examples include common
drill sizes and traditional tools used in the United States, U.S. standard
fastener sizes, commercial pipe sizes, and other common terms used in the
trades, the professions, the marketplace, sports, and various social activities.
When such descriptive information is given, values in acceptable units are not
required. For example, it is permissible to refer to a "36-inch
pipeline" or a "half-inch drill" without first giving the value
in an acceptable unit.
