It is critically important to define each melakha precisely and establish what is rabbinically prohibited and what is prohibited by Torah law, and moreover these determinations have practical consequences. For example, when writing is necessary in a hospital or in the army in order to save lives, le-khatĥila one should minimize the prohibitions one violates and write in a manner that is only rabbinically prohibited. We shall first clarify what is prohibited by Torah law and what is rabbinically prohibited, and then detail how one should write in cases where there is danger to human life.
The Torah prohibition of Kotev refers to writing normally with the right hand, though one who writes irregularly with the left hand violates a rabbinic prohibition. A lefty who writes with his left hand violates Torah law, and if he uses his right hand he violates rabbinic law. One who is ambidextrous violates Torah law by writing with either hand (Shabbat 103a).
If one holds the pen with a shinui – in his mouth, with his foot, or with the back of his hand – he violates a rabbinic prohibition (MT 11:14).
The Torah prohibition of Kotev also refers to writing that lasts for a significant amount of time. Therefore, if one writes with a pencil or pen on paper, he violates Torah law. However, if he writes using fruit juice, which will quickly fade, or with a regular pen but on a leaf that will dry out and crumble, he violates a rabbinic prohibition.
According to the vast majority of poskim, the Torah prohibition of Kotev applies to all languages (MT 11:10; BHL 306:11). A few poskim maintain that the Torah prohibition applies only to letters that may appear in a Torah scroll, but writing in any other script, in a foreign language, or in cursive Hebrew, constitutes a rabbinic transgression (Or Zaru’a).
If writing is necessary to save lives, and it is clear that a slight delay will not cause any further danger, the prohibition should be minimized by writing with a shinui, using the left hand. If one is ambidextrous, he should grasp the writing utensil with the back of his hand or between his little finger and ring finger. Ideally, a medical professional should buy a “Shabbat pen” whose ink disappears within a few days and with which writing is only rabbinically forbidden. If something must be typed on a computer, a shinui should be used if possible, such as typing with the knuckles, with a teaspoon, or the like. It is also preferable to write in cursive rather than in block letters.
To summarize, if one must write because of a life-threatening situation, it is preferable to use a Shabbat pen, since everyone agrees this is only rabbinically prohibited. Even with a Shabbat pen, it is proper to write using a shinui if possible. When a Shabbat pen is not available, one may write or type as needed, but in cursive and using a shinui when possible.