How to make a abbreviation of English words
For example, if I want to write the abbreviation of "Part Number", should I write it as "PN" or “PN." or "P.N." or "P/N"?

Is there any rules of the abbreviation?

Question

How to make a abbreviation of English words
For example, if I want to write the abbreviation of "Part Number", should I write it as "PN" or “PN." or "P.N." or "P/N"?

Is there any rules of the abbreviation?

Answer

Yes,  simply "PN" would be an abbreviation of the word you provided and the other examples may be acceptable too. However an abbreviation has meaning only when it is universally understood or within a particular group, so it doesn't always make sense to abbreviate everything.