Shabbat 103 - One writes two letters
When one plows on Shabbat, he is liable for any amount of plowing. Although with sowing there are amounts required for violation, such as two seeds - this is because people usually won't trouble themselves to sow one seed. However, with plowing, any amount of preparing the earth is significant.
If one is cutting branches from a live tree, then if he is doing it to improve the tree - any amount of cutting is significant. However, if his purpose is to collect firewood, he is liable once he collects enough to cook a "light egg" - that is, an egg of a chicken, which is the easiest to cook.
If one writes two letters, whether with his right hand or left hand, in any language, he is liable. Rabbi Yose say that the prohibition extends even further, and that any two useful marks, not letters, already make one liable. Others say that on the contrary, the two letters must be part of a word. Another question arises when one intended to write a long word, like Daniel, and stopped after writing the first two letters, dalet and nun, thus forming the word "Dan." Even though he did not complete his intent (the usual condition in Shabbat violations) , Rabbi Yehudah holds him liable.
Art: Gerard Ter Borch - Lady Reading a Letter