What is the most correct form for 3 in ordinal form: 3rd or 3d? I know both are valid. But I heard that 3rd is something like spoken form and it's grammatically correct to use 3d.
closed as general reference by tchrist, FumbleFingers, Kristina Lopez, MετάEd, kiamlaluno Mar 16 '13 at 11:31
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It's normal to represent third as 3rd, not "3d". The latter might be used if a foreign language were being translated by someone who is used to using only a single character to represent an ordinal (for example, in Italian).
This is supported by a Google Ngram comparing 3rd to 3d. The former occurs more often, even though "3d" could have a meaning as "three-dimensional". The large number of occurrences from the nineteenth century are probably explained by "3d" being "three pence", a currency amount so common that it had its own coin from around 1550 to 1967.
To be argumentative about the earlier centuries I googled "1st 2d and 3d" ...
A map from 1863, titled:
volume in the Library of Congress
and many more...