Plain Scale:
* How it's used: A plain scale is used to represent a single unit of measurement (e.g., 1 cm on the scale represents 1 meter in reality). It's essentially a straight line divided into units and sub-units. You measure distances directly on the scale and transfer them to the drawing.
* Example: A scale of 1:100 means 1 cm on the drawing represents 100 cm (1 meter) in reality. You'd directly measure the required distance on the scale and use that length on your drawing.
* Precision: Limited to the smallest subdivision marked on the scale. If the scale is divided into centimeters and millimeters, you can only measure to the nearest millimeter.
* Best for: Simple drawings where high precision isn't crucial.
Diagonal Scale:
* How it's used: A diagonal scale allows for greater precision than a plain scale. It uses a diagonal line to represent fractions of the smallest unit on the plain scale. This is achieved by creating a small square with sides representing the smallest unit and drawing a diagonal across it. The intersection of the diagonal with vertical lines represents fractional units.
* Example: A diagonal scale might allow you to measure to the nearest 0.1mm, even if the smallest division on the plain scale is 1mm. You would read the whole number of millimeters on the main scale and then the tenths of a millimeter using the diagonal scale.
* Precision: Higher precision than plain scales because it allows measuring smaller fractions of the smallest unit.
* Best for: Drawings requiring greater accuracy, such as detailed architectural or engineering plans.
In short: Choose a plain scale for simpler drawings where approximate measurements suffice, and a diagonal scale when high accuracy is needed to represent smaller units than can be conveniently marked on a plain scale.