Shabbos, Chapter Twelve, Mishnah 4. The Mishnah opens by establishing that one who writes two letters "be'helem echad" - in a single lapse of awareness - is liable. That is, a person who forgot that it was Shabbos, or forgot that writing is one of the labors forbidden on Shabbos, and within that very lapse wrote two letters, is liable.
This raises a question: what is the law when the person remembers in between one letter and the next? On this point there is a dispute, which we will learn in several of the coming Mishnayos.
The materials with which the writing is done:
The Mishnah lists the materials with which one who writes is liable:
"bidyo" - ordinary ink, the kind used for writing Torah scrolls.
"besam" - a dye made from a yellowish golden earth.
"besikra" - a reddish dye, a sort of red ink.
"bekomos" - a type of tree resin that was commonly used for writing.
"bekankantom" - copper sulfate.
"uvechol davar shehu roshem" - and with anything that leaves a mark; that is enough to be liable for writing.
Two letters in two separate places:
The Mishnah continues and discusses one who writes two letters that are not side by side on one surface, but rather in two places:
On the two walls of a corner, where they meet one another.
On the two facing pages of a ledger.
In both cases the rule is the same: if the letters are adjacent to one another and are read together, he is liable.
One who writes on his skin:
One who writes on his skin is liable. But what is the law when he does not write with ink on his skin, but rather scratches the letters into his flesh, such as when he takes a needle and scratches two letters with it? On this the Tannaim disagreed:
Rabbi Eliezer - holds him liable, since this too is a form of writing.
Rabbi Yehoshua - exempts him, because this is not the normal manner of writing, and therefore one is not liable for it on Shabbos.
In summary: in this Mishnah we learned the measure that creates liability for the labor of writing - two letters in a single lapse of awareness; the materials for which one is liable (ink, sam, sikra, komos, kankantom, and anything that leaves a mark); the rule that letters written in two separate places combine when they are read together; and the dispute between Rabbi Eliezer and Rabbi Yehoshua regarding one who scratches letters into his flesh.