Unit symbols are not followed by a period unless at the end of a sentence.

Unit symbols are unaltered in the plural.

The names of all units must be written in lower case when written in full form.

The symbols of units derived from names of scientists are written in upper case (only the first letter; eg.: N). The symbols of units not derived from names of scientists are written in lower case (eg: s).

The symbols of units cannot be written in plural (i.e. plural of kg is not kgs).
Rule1- Lower case is used to write the names of units.Units named after the scientists too are written in lower case,for example,newton,metre,kelvin etc.

rule2- Symbols of units that bear the name of the scientists are written in upper case.
       kelvin- K
       joule- J
rule3- Other symbols not named after scientists are written in lower case.
       Example- symbol for second- s
                      symbol for metre-  m
                      symbol for kilogram- kg
                      symbol for gram- g
rule4- Symbols of units are never followed by a full stop.
                      Symbol for kilogram is kg (not kg.)
                      Symbol for centimetre is cm (not cm.)
          However, when a sentence ends with the symbol of a unit,it is followed             by a fullstop.
rule5-  Units are written in plural only when used in words, but symbols remain             unaltered.
          For example:
                              symbols for 5 metres is 5m (not 5ms)
                              symbols for 3 kilograms is 3 kg (not 3 kgs)