I make my initial sketches by drawing lines in order, like line A, then line B, and so on. Then I go back and add value lengths to each of these lines. It often happens that after I give line A and value and then give line B and value it changes the value of line A. In order to prevent this from happening, I have to "Fix" line A so the length of the line can't change, than add a value length to line B and "Fix" that line length to prevent it from changing when I move on to Line C and give that line a length value. What am I doing wrong? Why doesn't the length of line A stay fixed when I change the length of line B?