Some time ago I looked at questions about trisecting an edge by compass and also straightedge, i beg your pardon entailed stating the rules for such constructions. Us left open another common question: Why space such build important, and why do we use those specific tools? This more than likely isn’t explained as frequently as it have to be.

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Why does that matter? Axioms

I’ll start with this question from 1998:

The prominence of Geometry ConstructionsI am doing a report on constructions in geometry. Ns would favor to recognize why constructions space important. Ns realize the they difficulty us to use various tools but there should be much more to it climate that. So i was wondering if you could give me more of a reason why constructions room so important?Since numerous things us ask kids to carry out are greatly to gain them provided to specific ways to usage their hands or bodies, it is understandable that Kel would expect that us teach build just because compasses and straightedges space worth knowing how to use. But that isn’t really it. Ultimately, it’s since our minds space worth knowing exactly how to use!

I answered:

Hi, Kel. That"s a an excellent question. We often tend to teach it the end of tradition, and also forget come think about why it"s worth doing!Certainly learning just how to use the devices is useful. Several of the approaches are advantageous in construction (of buildings, furniture, and so on), despite in truth sometimes over there are easier techniques builders use that us forget come teach. But I think the main reason for finding out constructions is their close link to axiomatic logic. If girlfriend haven"t heard the term, I"m talking about the totality idea the proofs and careful thinking that we frequently use geometry come teach.I’ve supplied compass constructions when I aided renovate a church building; however then the “compass” was a length of string. It to be the idea behind it that really mattered.

Euclid, the Greek mathematician that wrote the geometry text supplied for centuries, stated countless of his theorems in terms of constructions. His axioms are closely related come the devices he supplied for construction. simply as axioms and postulates let united state prove everything with a minimum of assumptions, a compass and also straightedge let united state construct whatever precisely through a minimum that tools. There room no approximations, no guesses. For this reason the an abilities you require to figure out exactly how to construct, say, a square there is no a protractor, are closely related come the thinking an abilities you should prove theorems around squares.A building is, at root, a theorem: If you monitor this sequence of steps, the result will have to be the object you claim to it is in creating, such together the bisector of one angle, or a triangle the meets certain requirements. So finding out to design a building and construction is practice in “constructing” geometrical proofs. Practice in building and construction is not mostly practice in using your hands, but your mind.

I closeup of the door my quick answer by referring to the very first proof in Euclid’s Elements, Proposition I.1:

Proposition 1


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It is forced to construct an it is intended triangle ~ above the directly line AB.

Describe the circle BCD with center A and radius AB. Again describe the circle ACE with center B and radius BA. Join the right lines CA and CB from the point C at which the circles reduced one another to the points A and B.

Now, because the point A is the center of the circle CDB, therefore AC equals AB. Again, because the point B is the center of the circle CAE, therefore BC equals BA.

But AC was proved equal to AB, therefore every of the directly lines AC and BC equals AB.

And things which equal the very same thing likewise equal one another, therefore AC also equals BC.

Therefore the three right lines AC, AB, and BC equal one another.

Therefore the triangle ABC is equilateral, and it has actually been constructed on the provided finite directly line AB.


This theorem is in truth a construction. Keep in mind that the measures involve making circles (with a compass) and also making present (with a straightedge); and at the end he place “Q.E.F.”, brief for “Quod erat faciendum”, Latin because that “Which was to be done”. (Euclid, the course, actually supplied Greek, “ὅπερ ἔδει ποιῆσαι”, “hoper edei poiēsai”.)

Why not rulers and also protractors? Axioms

In 2002, we acquired a similar question native a teacher, that referred to as for a little much more detail on exactly how the axioms (Euclid’s Postulates) relate to the compass and also straightedge:

Why Straightedge and also Compass Only?My Geometry students want to know why constructions have the right to only be done using a straightedge and also a compass. They want to understand why lock can"t just measure a heat segment to copy it or usage a protractor to build an angle. What"s the difference? We have actually searched ours book and some web sites include constructions, but to no avail.I referred ago to the ahead answer, then elaborated.

There room two ways that I deserve to see to define the border rules because that constructions, which pertained to us indigenous the old Greeks:1. Lock are simply the rule of a game mathematicians play. Over there are plenty of other ways to perform constructions, however the compass and straightedge were favored as one collection of tools that make a construction challenging, by limiting what you are enabled to do, just as sports restrict what you can do (e.g. Touching but not tackling, or tackling but no nuclear weapons) in order to keep a video game interesting. Various other tools could have been liked instead; because that example, geometric constructions can be done utilizing origami.Euclid might have began with any tools he wanted; but a major goal was to border what could be done, as kind of a video game to see how small we have the right to use, come do just how much.

For an ext on axioms or postulates, watch my series in July 2018, beginning with Why does Geometry begin With Unproved Assumptions?

But it’s not simply a game; it’s the game:

2. They are the communication of one axiomatic system, through the score of ensuring the geometry is developed on a solid foundation. Euclid want to begin with as couple of assumptions as possible, therefore that all of his conclusions would be details if friend just accepted those few things. So he listed five postulates (in enhancement to some other presumptions even more basic); I"ve taken this from the reference provided in my answer above: Postulate 1. to attract a straight line indigenous any suggest to any kind of point. Postulate 2. to create a finite right line repetitively in a straight line. Postulate 3. to define a circle with any kind of center and also radius. Postulate 4. That all right angles same one another. Postulate 5. That, if a directly line fall on two right lines provides the internal angles top top the same side less than two appropriate angles, the two straight lines, if developed indefinitely, fulfill on that side on which space the angles much less than the two right angles.He starts v the existence of lines and circles, climate adds just two extr facts. (His system is not fairly complete, and extr axioms room now recognized to it is in necessary.)

The an initial two postulates say that you can use a straightedge: heat it up v two offered points, and also draw the line in between them, or heat it up v an present segment, and draw the line beyond it. That"s the very first tool friend are allowed to use, and also those are the only methods you are allowed to use it.As is frequently noted, you are not permitted to do various other things, choose measure or copy a length by making clues on the straightedge. This is no just since Euclid want to save his tools clean! It’s due to the fact that he want to minimization his assumptions, prove as much as possible starting with as tiny as possible.

The 3rd postulate claims you deserve to use a compass to attract a circle, given the center and radius (or a allude on the circle). The is the only means you are allowed to usage the compass; girlfriend can"t, because that example, attract a circle tangent come a line by adjusting that is radius till it _looks_ tangent, without discovering a particular point the circle needs to pass through.The last 2 postulates relate come angles, and also are less connected with the construction process itself than through what girlfriend see when you space done.Again, the constraints are to minimization the assumptions, not since his compass to be defective. I actually understated the restriction in this case. (More on the later.)

So yes, really the two devices Euclid required for a building just represent the assumptions he to be willing come make: if these two tools work, then you deserve to construct everything he speak about. Because that example, you deserve to use this tools, in the prescribed manner, to build a tangent to a provided circle v a given point; but it take away some assumed to uncover how to do so (without just drawing a line the _looks_ tangent), and it takes number of theorems to show that it really works.Here we are ago to the challenge! and also the score is not simply to do something the looks right, yet to have the ability to prove something.

Of food you deserve to just measure a line or one angle, if her goal is just to make a illustration - and also usually that will certainly be an ext accurate 보다 a complicated compass construction! but when friend use just the tools allowed in this game, you space actually playing within an axiomatic system, getting a feel for exactly how proofs work. Friend are at the same time playing a challenging game, and also doing one of the couple of things in life the can offer you pure certainty: if these lines and also circles were exactly what they pretend to it is in (with no thickness, etc.), then the allude I construct would be specifically what I claim it is. And also it"s that sense of certainty that the Greeks were looking for.

Why go the compass collapse? Axioms!

I didn’t mention over a unique restriction top top the compass, which turns out come be entirely theoretical. We acquired a question around that in 2003:

Collapsible CompassI require to know what a collapsible compass is and what that is supplied for. Every I know is that once you choose it increase from the paper, you shed your place.Again, ns answered the question, maintaining it brief:

The collapsible compass is no something the is "used"; rather, it to represent the truth that Euclid want to do as couple of assumptions (postulates, or axioms) at the basic of his proofs as possible. So rather than assume that it was possible to relocate a line around, maintaining the same length (as you could do through a real, addressed compass), or equivalently the you can attract a circle v a offered center and length, he assumed only that friend can draw a circle v a offered center and also through a offered point. Then he went on come prove the if you can do that, you could then build a circle through a provided radius, or relocate a line to a given place: Collapsible Compass http://mathforum.org/library/drmath/view/52601.htmlThe referral is come a quick answer that web links to the proposition ns am around to discuss.

Where ns quoted Euclid’s postulates above, it may look as if you can just set the compass to any type of radius girlfriend want, contradictory to what I’ve said here: “ to describe a circle with any type of center and radius.” but the indigenous “radius” come Euclid walk not refer to a number, together we think of it today, but to a specific segment! This is made explicit in the commentary to the elements on Joyce’s site that I’ve described before, Postulate 3:

Circles were identified in Def.I.15 and also Def.I.16 as aircraft figures v the residential property that over there is a certain point, referred to as the facility of the circle, such that all directly lines from the center to the boundary room equal. The is, all the radii space equal.

The offered data room (1) a allude A to it is in the center that the circle, (2) another allude B to be on the circumference that the circle, and also (3) a airplane in which the 2 points lie. …

Note the this postulate does not permit for the compass to it is in moved. The usual means that a compass is used is the is is opened up to a offered width, climate the pivot is placed on the drawing surface, climate a circle is attracted as the compass is rotated approximately the pivot. But this postulate go not permit for transporting distances. It is together if the compass collapses as quickly as it’s gotten rid of from the plane.

See more: Wh At What Level Does Nidorino Evolve, What Is The Best Level To Evolve Nidorino

Proposition I.3, however, provides a construction for moving distances. Therefore, the very same constructions that have the right to be made through a continual compass can additionally be made with Euclid’s collapsing compass.