Introduction
Carpenters, engineers, and architects use different tools that give accuracy and perfection to their work. This may be for taking measurements, drawing lines or different geometrical shapes, or determining straightness or curves in an object. Hence, every part must have its accurate value parameters and match relative to each other, especially in these fields. This is very important for a perfect assembly, for perfect breakthrough innovation, or the design of a product newly manufactured.
Two commonly observed instruments of use in these fields are the straightedge and the ruler. Both, despite similar straight sides, may have varying applications and uses. A ruler is, in fact, a straight edge, but a straight edge cannot replace a ruler because it has measurements on it. This is the main difference that sets both these instruments apart in use. When the same geometric instrument ruler occurs without any marks, it becomes a straight edge that can be used to draw starlight lines between two points. It ensures accurate plotting of graphs and evenness of surfaces.
The use of straight edges may be limited, whereas rulers manufactured these days are apt to adapt to different conditions and fields of activity and are an essential utility in almost every scenario of daytoday life, including construction, carpentry, engineering, road works, mathematics practical's, geometry and many more.
Depending on the field of use, its applications can be variable and of highly efficient use. People who need perfection and accuracy in their projects are among those who commonly benefit from these easytouse but efficient instruments that were invented long ago. Both these instruments have their use, and it is necessary to not replace them.
Straightedge vs. Ruler
A straightedge is used by architects, or professionals in industrial and construction fields, mainly for the purpose of ensuring a flat surface without any curves, bumps, or interruptions and that the surface is even. This instrument has cumulative effects later on the final outlook of a project, small or big, when all put together. It does not evaluate using any kind of measure or gradients.
An additional advantage of two straight edges in its structure is that it provides users in drawing straight or parallel lines. This is an added benefit compared to the straight edge, whose uses are only limited. On the other hand, rulers used varyingly from school kids to high scholars and college students may not actually be of professional use, but only a much simpler use of measuring in using different parameters.
Differences between Straight edge and Ruler in tabular form
Main parameters considered for comparison  Straight edge  Ruler 
1. Appearance and features  A single edge  Two parallel flat edges with multiple measurements. It is also referred to as a ‘Scale’, ruler gauge, or line gauge. 
2. Areas in use  More professional in Constructional, architectural uses in drawing plans.  A much simpler instrument which does not require professional knowledge. Mathematics, Geometry, 
3. Application  Determine the surface’s flatness and ensure absence of any bumps or curves.  Take measurements, and draw straight lines. 
4. Similar instruments in use 
Compass is an example of a similar instrument.
Often used with a straightedge in drawing plans.

Protractors, Set Squares, and dividers are frequently sold as part of a set, which is commonly referred to as an instrumental box.

5. Advantages  To determine the flatness, curve, or shape of surfaces. 
Measurement is possible in multiple gradients, including centimeters, millimeters, and inches on either side of the ruler. Also wide varieties are made available in different shapes, sizes and flexibility suitable for different purposes.

6. Disadvantages  Expense is much higher on these sets of instruments as they have professional applications and require primary knowledge about their use. 
It is easy to use and measurements are made possible in multiple parameters without conversion requirements.

7. Costs 
More expensive and a professional instrument of use.
 Cheaper and common among school, college students, engineers, road workers, carpenters construction fields, and others. However, it may depend on the material it is made from and its application field. 
What is a straight edge?
The compass and straightedge are more important in constructing geometric structures, and their uses extend beyond schools and colleges and are more of a professional utility. A straightedge ensures if a surface is even or not. The application of a compass and straightedge seen at first glance cannot be grasped easily by a person new to the field and may require some assistance to grasp its working principles. It does not have any measurements on it, unlike a ruler.
To cut a straight line and cut out edges, you must measure your object along a side. Then, move to the opposite edge of your material, and mark the same measurement on that side (thereby making an imaginary straight line connecting both). Line up your straight edge, and drag a straight line between the marks obtained. You should have got a perfectly straight line cut by now. Often provides better finishing details. You can also use it as a guide to lean against as you cut along using a knife, scalpel, or another craft blade. This also protects your fingers and at the same time gives a cleaner finish.
What is a ruler?
A ruler can be used to measure how long an object is or draw different geometrical shapes using straight lines like a pentagon, hexagon, etc. It is more useful in simple applications and at the same time, efficient in professional applications due to the wide variety of rulers made available nowadays in the market. In different shapes, and sizes and made of different material apt for different conditions it demands. For example, a carpenter uses a long wooden scale that can measure about thirty to forty inches. Also, being an easytouse instrument, it allows one to calculate its parameters in multiple values, like in centimeters, millimeters, and inches displayed on either side of it.
It is a device used to measure and draw straight lines at the same time.
It can be used to determine the lengths in metric or customary units of measurement.
Rulers manufactured these days could be made of wood, plastic, or steel. This can also be used to cut out along the straight edges or straight lines.
For example, a piece of paper cut perfectly along a straight line. It is also commonly used by engineering students, who often prefer to use steel scales as their surface remains close to the object's surface and hence could provide more accurate measurements, which is very important for the values they must determine.
A much longer wooden scale of about thirtysix inches is used in carpentry and has metric measurements. Rulers can also be used in construction or architectural work, and their names may differ, but the basic working principle remains the same.
To measure the length of an object, the object is placed such that its front end overlaps the zero of the scale, the total length it takes is then observed, and the last line of the scale is taken so that it coincides with the object. The length will be indicated by the mark measurements. If the object does not coincide with zero on the scale initially, then the distance between the initial and final points will be the total length of the object. Different units can be used based on the side of the scale used.
The long hash marks appearing on the ruler stand usually for a centimeter. A much smaller unit is denoted by the smaller hash marks in between the bold and large hash marks. Ten millimeters make up a centimeter.
Some other uses that can be mentioned are carpentry, cutting straight lines, and engineering, design, or plan drawings.
Differences between a straight edge and a ruler in points
Main Function
A straightedge determines the flatness or straightness of an object and is often used to ensure the primary assembling parts are perfect concerning each other as required to assemble them all into one piece. These may be plates, wooden planks, or any other
A ruler can define the measurements of an object in centimeters or millimeters on one side of the ruler and inches on the other side of the ruler. It could also tell us about the threedimensional volume of a structure when measured by calculating separately the length, breadth, and height. This can be used to ensure the drawing of straight, accurate lines. The proper measures may also be necessary while drawing graphs and plots or geometry, where we may need to find the intersection of two lines.
Areas of application
Straightedge use is observed in automotive service and machining industries to check flatness, for example, a plan or any other object so that the warping in a piece of wood could be much easier.
Rulers are used by varying ages, from primary schools to mathematics and geometry colleges, and are a basic instrument that makes proper measurements of every dimension of an object or shape.
Costs
Straight edge may be relatively expensive due to its professional adaptability and utilities. Typically purchased by mechanics, architects, and in the automobile industry, among others.
Uses
Rulers, despite the same principle being used in wide areas, may differ in names. It ranges from architectural work to engineering, construction, geometry, mathematics, and many more. The appearance may also be different and more appropriate for particular conditions. For example, the use of a wooden scale that is long enough up to thirtysix inches, steel rulers used by students in engineering so that the plane of work and scale remain nearby, and the measurements made are more accurate, just like it demands the accuracy in values taken for a project.
The varying adaptability of a straight edge makes it more of a professional tool, whereas a ruler is a much simpler and more common measuring instrument.
Conclusion
Both rulers and straight edges are common instruments in use. However, the only difference is in its area of application and extensibility of use in various fields.
The usage of straight edges and rulers demands a different range of knowledge about how to use them. For example, a straight edge could only be of use by a professional with previous knowledge or experience. Whereas, rulers most commonly used in geometrical works are much easier to understand how they work. It has only one application on paper, whereas straight edge limits extend beyond a single view to dimensions and surfaces.
Nowadays, mathematics curricula emphasize the use of the compass and straightedge.
For any job, the right choice of instrument is the first step to be taken. After you've decided what to use, the next step is to understand how it works. Sometimes a ruler can be called a rule, or a line gauge. Rulers replaced the primitive human measurements using hands, feet, and ounces.
This instrument was first invented in Germany and then, later on, led to multiple inventions into different types of rulers in shapes and sizes. It is usually a metal or plastic instrument, often light in weight. The metallic one is more alt in terms of construction as it lies still and more closeness to the object is measured. It ranges from yardsticks (wooden or plastic), surveyors' scales, and tape, all with a similar purpose. We can also use rulers that are flexible and can be curved. A straightedge is also used to help draw accurate graphs and tables.
A ruler and compass construction refer to construction using an unmarked ruler and a compass. With the help of a ruler and compass, one can bisect an angle into two equal parts. However, it can be proved that an angle cannot be divided into three equal parts using only a compass and straightedge that is, angle trisection. However, should two marks be allowed on the ruler, the problems are resolved?
Depending on the size, shape, or material it is made up of and its possible applications, the expense of the instrument will vary.
References
 https://en.wikipedia.org/wiki/Straightedge