Shabbos, Chapter 12, Mishnah 3. This mishnah deals with the labor of writing on Shabbos and with the measure that makes one liable: writing two letters.
One who writes two letters is liable, and this includes:
"bein bimino bein bismolo" - whether with his right hand or with his left. This speaks of someone who can write with both hands, and therefore it makes no difference whether he wrote with his right or with his left.
The same letter twice - such as two alephs or two beis. And some understand that it refers specifically to two letters that together form a word, such as "gag" (roof) written with two gimmels, or "vav".
Two different letters - even though they are not two identical letters, he is liable.
Two kinds of ink - even if the two letters were written with two different kinds of ink.
In any language - liability does not depend on the language in which they were written, as long as he wrote two letters.
The view of Rabbi Yosi - marks are enough:
Rabbi Yosi holds that liability for writing two letters is based on the very essence of writing, namely making a mark, and not necessarily on forming an actual letter. Even one who makes two marks that correspond to one another is liable. The source for this is the construction of the Mishkan: on the beams they would regularly inscribe letters or shapes, so as to know which beam stood next to which.
This is comparable to a succah whose boards are numbered, where the numbers tell you which board goes alongside which. In the Mishkan they were careful to preserve the order of the beams and to set each beam in its fixed place, and for that purpose they marked each pair of adjacent beams with the same letter or the same sign. From here we see that a mark alone suffices, and it need not be an actual letter, as Rabbi Yosi says.
The view of Rabbi - a small name within a larger one:
Rabbi adds that even one who writes a small name that is part of a larger name is liable: a person who intended to write the larger name, and because he stopped in the middle (for example, he was interrupted) ended up with only the small name. For example:
"Shem" out of "Shimon" - he intended to write "Shimon", stopped, and was left with only the word "Shem".
"Shem" out of "Shmuel" - and the same law applies if he intended to write "Shmuel" and produced only the first two letters.
"Noach" out of "Nachor" - he intended to write "Nachor", and wrote only a nun and a ches.
"Dan" out of "Daniel".
"Gad" out of "Gadiel".
In all these cases he is liable, since the small name is itself a name in its own right and is made up of two letters, and therefore he becomes liable for it, even though his original intention was to write the larger name.
In summary: in this mishnah we learned that the measure for the labor of writing is two letters - with the right hand or with the left, the same letter twice or two different letters, with two kinds of ink and in any language. Rabbi Yosi teaches that even two marks suffice, just as the beams of the Mishkan were marked so that each beam could be set in its place; and Rabbi adds that even one who writes a small name while intending to write a larger name is liable, for the small name is a complete name of two letters.