One short stroke on the top and one long upward curved stroke on the bottom. The top short stroke indicates the sky, the bottom curved stroke indicates the earth. The short stroke over the upward curved stroke means Up.
The shell bone script for character Down is
The small seal script for Up is
the bottom horizontal line indicates the earth, and the vertical half straight and half curved line indicates Up, the short horizontal line is the indicator - "I mean 'here'".
The small seal script for Down is
The clerical script for Up is
The clerical script for Down is
The current standard printing script for Up and Down are
They are called indicative characters. The short line, or the diagonal short line or pointer are the indicators to tell the meanings of the characters.
The Pinyin of 上 is Shang1, and Pinyin for 下 is Xia4.