The Title of Earl

I am sure that I am not alone when it comes to reading history that it is very confusing when an Earl is referenced in a text. While an Earl is well defined what is not is that there are often multiple creations of an Earldom and most texts assume, or appear to, that you know what creation they refer.

In addition, it seems to be a shirt form and it obviates the use of the full name of the Earl in question. In my research I refer to the Earls of Essex and Oxford quite extensively. Oxford is possibly easier as the Earls of Oxford are normally a de Vere of some generation. Essex and other earldoms are not quite so straight-forward.