In aligned dimensioning, the dimension text is placed parallel to the dimension lines. This means that horizontal dimensions are read from the bottom of the drawing sheet, while vertical and angled dimensions are read from the right-hand side of the sheet. To read vertical dimensions, you would need to rotate the drawing 90 degrees clockwise. This system is often used in architectural and structural drafting because it keeps the dimensions aligned with the features they describe. 

In unidirectional dimensioning, all dimension text is oriented horizontally, so it can be read from the bottom of the drawing sheet without any rotation. The dimension lines are typically broken to insert the dimension value. This system is the most common standard in mechanical drafting because it is easier to read on large or complex drawings, such as those for aircraft or automobiles, where rotating the drawing would be inconvenient.