Yevamot 84 - Review
Based on previous discussions, Talmud formulates a rule: a woman may be permitted to her husband but forbidden to her brother-in-law (yavam) in case of the husband's death; she may be permitted to the yavam but not to the husband; she may be permitted to both; and she may be forbidden to both.
For example, if a woman was a widow, and a regular Kohen married her, but his brother is a High Priest (Kohen Gadol), then the brother would not be allowed to do a yibum - since a High Priest should not marry a widow - and thus she is permitted to her husband but not to the yavam.
An example of the second category: one can construct an example with a High Priest, but a less trivial one is a regular Israel who married a woman who is illegitimate (mamzeret) and who has an illegitimate brother (mamzer). A regular Israel is not allowed to marry a mamzeret, but a mamzer is, thus we have a case of a woman who is forbidden to her husband but is permitted to her yavam.
A new example: a High Priest married a widow. This was a prohibited marriage, so the woman becomes disqualified (chalalah), and now she cannot marry any Kohen, not because she is a widow, but because she is a chalalah. This is a situation where she is forbidden both to her husband and to her yavam.
The fourth category includes all other combinations, even though the Talmud finds cases which are not listed in the first three categories and yet do not belong to the fourth, which means that the examples lists are not exhaustive.
Art: The Widower by James Jacques Joseph Tissot