#Were
> Jonathan Oldstyle and Diedrich Knickerbocker were two of Irving's pen name<b><i>s</i></b>

**Note:** use the plural *names*, rather than <code>*name*</code>, because you are talking about a collective of pen names, by saying <code>*two of*</code>, you are implying that there are many (more than one) pen names.

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**Were** is (in this case) the 1st and 3rd person, **plural** past form of the verb *be*.

**Was** is (in this case) the 1st and 3rd person, **singular** past form of the verb *be*.

You are referring to <code>*two pen names*</code>, so the **plural** form is correct.

Here is an example where the singular form, **was**, would be correct:

> Jonathan Oldstyle was one of Irving's pen names.