The letter "A" is actually a pyramid but it can also be interpreted or symbolized as two important pillars. This is why the word "America" has two "As" in the beginning and at the end, take for example > "AmericA". The two "As" also represent the two pillars of Solomon's temple. The ancient Egyptians called these two pillars "Tatt (Sutukh)" and "Tattu (Haru)," and in Freemasonry they are called "Boaz" and "Jocktin". So the letter "A" is very important, and its origin can be found in Sumeria/Mesopotamia. The letter "A" is derived from the image of a bull's head; if you flip the letter "A" upside-down, that's exactly what you get -- an image of a bull's head.
Moreover, I want to note that if you remove the top of the letter "A", it will look just like the pyramid on the one dollar bill. More importantly, the letter "A" represents the beginning or something unfinished. The pyramid on the one dollar bill, for example, is unfinished. Now we can see clearly as to why the letter "A" is one the most important characters on the dollar bill.