Instructables
Picture of handy tricks to find square

The following instructable details 2 ways to find out if something is square, 1 way to draw an accurate perpendicular line, and 1 way to draw an accurate parallel line.
These tricks involve virtually no math to do and are scalable to any dimensions from millimetres, to miles.
The 4 tricks are probably not something you will use every day but are really handy to know and will allow you to amaze others with your skill and mental prowess.
I have embedded the video podcast of this because it is sometimes easier to see it done than to try to understand a written description. my part is at 5:24  Don't be shy about watching my other episodes or checking out my other instructables.

 
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Step 1: Trick #1 - the 3,4,5 trick

Picture of Trick #1 - the 3,4,5 trick

Note* this trick is the only one that uses math and it is only really to explain how it works. So don't be scared off.

 

According to Pythagoras, a right angle triangles sides can be described by the equation

a squared + b squared = c squared

where c= the hypotenuse (the longest side).
By a lucky fluke of math 3 squared + 4 squared just happens to = 5 squared. So all you have to remember is 3,4,5. Simple so far right?

 

If you have something you are working on, for example a wooden frame, cut plywood, a tacked metal structure or even brickwork, and you want to see if it is square, this trick is for you. Just measure 3 units across from the corner and make a mark, then 4 units up and make a mark. Now measure the distance between the two marks and you should get 5 units. If not then it is not square. If it is more than 5 units it is more than 90 degrees. If it is less than 5 units it is less than 90 degrees. I am saying units because it does not matter what you use to measure, what matters is that the 3,4,5 ratio is correct. Just know that the longer measurements you can take, the more accurate it will be. You can even do multiples of the ratio, for example 12”x16”x20”.

streetrod53 years ago
Trick # 2 - Had a real life experience with this one. Measuring both diagonals will work ONLY if you prove at least one corner square (3-4-5 trick), OR you know your top = bottom and left = right. If you have a trapezoid (left = right, but top <> bottom), your diagonals will still be equal. Thanks for Trick 3 & 4, and for spring-making -ible.
zieak5 years ago
A nice collection for the mental tool box.  I always seem to forget which three consecutive numbers it is for the 3,4,5 trick.  I'll try and remember that it ends with the number of fingers on one hand since it is a "handy' mnemonic.  I'll also hope i don't lose any fingers.
baudeagle zieak5 years ago
Here are some more Pythagorean triples if you are interested.

( 3, 4, 5) ( 5, 12, 13) ( 7, 24, 25) ( 8, 15, 17)
( 9, 40, 41) (11, 60, 61) (12, 35, 37) (13, 84, 85)
(16, 63, 65) (20, 21, 29) (28, 45, 53) (33, 56, 65)
(36, 77, 85) (39, 80, 89) (48, 55, 73) (65, 72, 97)

http://en.wikipedia.org/wiki/Pythagorean_triples


dave spencer (author)  baudeagle5 years ago
That's neat, thanks for posting that.  I'm surprised how many there are.  I would not have thought they were that common.
Phil B5 years ago
I once used the 3-4-5 trick to lay out starting lines for laying floor tiles in my basement.  To acquire greater accuracy, I doubled the numbers to make it the 6-8-10 trick. 
oddstray5 years ago
Right-angle tips (aka Square Tips) are very often useful!  Thanks for these ...

... and clarify step 3, please.  Do the two arcs come from the two arrows on the horizontal?  If so, then how and why ...


dave spencer (author)  oddstray5 years ago
Yes you are correct, sorry if I was not clear. The video shows it from start to finish and the picture in the introduction is actually a screen capture of the video from that trick. It may help clarify also.