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The
diagram above shows graphically how the 22 SI derived units with special names and
symbols are related to the seven SI base units. In the first column, the symbols of
the SI base units are shown in rectangles, with the name of the unit shown toward
the upper left of the rectangle and the name of the associated base quantity shown
in italic type below the rectangle. In the third column the symbols of the derived
units with special names are shown in solid circles, with the name of the unit shown
toward the upper left of the circle, the name of the associated derived quantity shown
in italic type below the circle, and an expression for the derived unit in terms of
other units shown toward the upper right in parenthesis. In the second column are
shown those derived units without special names [the cubic meter (*m*^{3})
excepted] that are used in the derivation of the derived units with special names.
In the diagram, the derivation of each derived unit is indicated by arrows that bring
in units in the numerator (solid lines) and units in the denominator (broken lines),
as appropriate.

Two SI derived units with special names and symbols, the radian,
symbol rad, and the steradian, symbol sr (bottom of the third column of the diagram),
are shown without any connections to SI base units – either direct or through other
SI derived units. The reason is that in the SI, the quantities plane angle and solid
angle are defined in such a way that their dimension is one – they are so-called dimensionless
quantities. This means that the coherent SI derived unit for each of these quantities
is the number one, symbol 1. That is, because plane angle is expressed as the ratio
of two lengths, and solid angle as the ratio of an area and the square of a length,
the SI derived unit for plane angle is *m/m* = 1, and the SI derived unit for
solid angle is *m*^{2}/*m*^{2} = 1. To aid understanding,
the special name radian with symbol rad is given to the number 1 for use in expressing
values of plane angle; and the special name steradian with symbol sr is given to the
number 1 for use in expressing values of solid angle. However, one has the option
of using or not using these names and symbols in expressions for other SI derived
units, as is convenient.

The unit "degree Celsius" which is equal to the unit "Kelvin"
is used to express Celsius temperature *T*. In this case, “degree Celsius" is
a special name used in place of "kelvin" This equality is indicated in the diagram
by the symbol K in parenthesis toward the upper right of the ^{o}C circle.
The equation below "CELSIUS TEMPERATURE" relates Celsius temperature t to thermodynamic
temperature *T*. An interval or difference of Celsius temperature can, however,
be expressed in kelvins as well as in degrees Celsius.