Gittin: Chapter 9, Mishnah 8. This mishnah opens with a list of gittin that are valid even though the language in which they were written, or the wording of the signatures, was not the standard one, and it closes with the law of a get given under coercion.
Valid gittin:
"Kesavo Ivris ve'edav Yevanis" - the get was written in Hebrew and the witnesses signed in Greek.
"Yevanis ve'edav Ivris" - the get was written in Greek and the witnesses signed in Hebrew.
"Ed echad Ivri ve'ed echad Yevani" - the get was written in any language, but one witness signed in Hebrew and his fellow in Greek.
"Kesav sofer ve'ed" - literally: the scribe wrote the get and one witness signed it. The Gemara explains that the meaning is that the signatures came from the scribe and from one additional witness.
What is the concern with the scribe himself signing? The husband did not ask the scribe to serve as a witness; he only instructed that two men be appointed to sign the get, along with someone to write it. His agents, when they came to tell the scribe to write the get, may have worried that he would feel slighted that the husband had not considered him worthy of serving as a witness, and out of embarrassment they would tell him to sign even though the husband never asked for this - and such a get is invalid, since the appointment of the witnesses must come from the husband. The mishnah teaches that there is no such concern: even when the scribe is one of the witnesses, we assume that the husband instructed him to serve as a witness, and the get is valid.
The wording of the witnesses' signatures:
"Ish peloni ed" - the witness wrote only his first name followed by "witness" - valid, and he is not obligated to write his father's name.
"Ben ish peloni ed" - he wrote that he is the son of so-and-so, without his own first name - valid.
"Ish peloni ben ish peloni velo kasav ed" - he wrote his own name and his father's name, but did not add the word "witness" - valid.
And this was the practice of the "nikiyei hada'as" of Jerusalem. "Nikiyei hada'as" is an expression whose literal meaning is those of clean and pure mind, in other words: people of special stature, who were precise and concise in their language. They would leave out the word "witness," because they saw no need for it - after all, the get is valid even without it.
And what is the law if, instead of the exact names of the husband and the wife, their nicknames were written, or according to an alternative explanation, their family name in place of their first name? This too is valid.
A get me'useh - a get given under coercion:
The status of the get depends on the identity of those applying the coercion:
"B'Yisrael - kasher" - if a Jewish court compelled the husband to give a get, the get is valid. The Gemara explains that they compel him until he says "I want to," since he must express with his own mouth that he wants to give the get.
"U'v'goyim - pasul" - if gentiles compelled him on their own initiative, the get is not valid.
The Rambam gives his famous explanation that although the coercion appears to run against the husband's will, deep in his heart a person wants to do the right thing, and it is only his evil inclination that stands in the way and tells him to treat his former wife badly. The coercion is in fact coercing the evil inclination, and it enables him to do what he truly wants in his heart of hearts. It emerges that the get is given with his genuine will, provided that he expresses this with his mouth after submitting to the coercion.
It is worth being precise about the parameters of coercion by gentiles: even when their coercion had a justified basis, meaning that halachically there was sufficient reason to obligate the husband to give a get, the get is not valid, but it does carry a "rei'ach ha'get," the scent of a get, and that is enough to disqualify the woman from marrying into the kehunah, so that no kohen may marry her afterward. But if the gentiles coerced him with no halachic basis at all, the get does not disqualify her even for the kehunah.
On the other hand, the court may make use of gentiles as the instrument of coercion: although coercion by gentiles acting on their own initiative invalidates the get, even where there is justified reason, they may be used as a stick in the hand of the court. The gentiles "chovtin oso ve'omrim lo: aseh mah she'Yisrael omer lach" - they beat him and say to him: do what the Jew tells you, that is, do as the Jewish court directs - and such a get is valid.
In summary: in this mishnah we learned that a variation in the language of the writing or in the wording of the signatures does not invalidate the get, and that even a get that the scribe wrote and signed as one of the witnesses is valid, since we do not suspect that his appointment did not come from the husband. We reviewed the valid forms of signature, the concise practice of the nikiyei hada'as of Jerusalem, and the validity of a get in which nicknames were written. Finally we learned the laws of a get me'useh: by Jews it is valid, since they compel him until he says "I want to," and by gentiles it is invalid, yet when the gentiles act as agents of the court and tell him "do what the Jew tells you" - the get is valid.