3, 4, 5 Triangle
Need a Right Angle (90°) ?
Make a 3,4,5 Triangle !
Connect three lines:
And you will have a right angle (90°) 


Other Lengths
You can use other lengths by multipying each side by 2. Or by 10. Or any multiple.
Drawing It
Let us say you need to mark a right angle coming from a point on a wall.
You decide to use 300, 400 and 500 cm lines.





Draw a 300 line along the wall 
Draw an arc 400 away from the start of the 300 line 
Draw an arc 500 away from the end of the 300 line 
Connect from the start of the 300 line to where the arcs cross 
And you have your "3,4,5" triangle with its right angle 
The Mathematics Behind It
The Pythagoras Theorem says:

The square of a (a²) plus the square of b (b²) is equal to the square of c (c²):
a^{2} + b^{2} = c^{2} 

Let's check if it does work:
3^{2} + 4^{2} = 5^{2}
Calculating this becomes:
9 + 16 = 25
Yes, it works ! 
Other Combinations
Yes, there are other combinations that work (not just by multiplying). Here are two others:


5,12,13 triangle 
9,40,41 Triangle 
5^{2} + 12^{2} = 13^{2} 
9^{2} + 40^{2} = 41^{2} 
But there are infinitely many more.... read Pythagorean Triples for more information.
