The index is 1, the middle finger 2 and the ring finger 3. That's the right hand.
The left's index adds 5 (for 6, 7, 8,...), and further combinations complete the numbers. For letters, you may need to employ the left middle finger, finally, and more intricate combinations, of course.
All in all, five fingers — two of the left and three of the right hand — are enough to send texts and numbers in binary (on or off) form over wires in Émile Baudot's code from 1870, possibly the first plain text character set. Today, you can have Mozilla Thunderbird show you on screen a descendants of that code — also in but plain text, even when the messages themselves contain superfluous and distracting formatting:
›› Fancy HTML formatting is nice, but it can also be a risk to privacy and security (and, honestly, some of the fancy formatting isn't worth it anyway). Fortunately, you can make Mozilla Thunderbird, Netscape or Mozilla display emails using plain text only. Here's how.