The Uighur alphabet was derived from the cursive Sogdian script, which ultimately traces back to Aramaic. However, unlike Sogdian and Aramaic, the Uighur script is written from top to bottom and in columns going from left to right, like Aramaic turned 90 degrees counterclockwise. In fact, even the letters themselves were rotated 90 degrees.
With a few exceptions, letters in Uighur have three variants depending on location within a word. At the beginning of a word, the letter can take on the initial form. At the end of a word, the final variant is used. And in all other places, the medial variant is employed.
The Uighur alphabet was adopted to write Mongolian in 12th century CE.