A speed square is a tool that just about every carpenter will have in her bag. It can be used to make lines perpendicular (square) to a board's edge. And it can be used to mark any angle (between 0 and 90 degrees) across a board's face. There are many other ways the speed square can be used, but I will focus on these two in this tutorial.
Why is a triangle called a square? Merriam-Webster defines a square as: a four-sided shape that is made up of four straight sides that are the same length and that has four right angles.
It is the four right angles that defines our tool. The speed square only has one right angle, but one of it's main purposes is to quickly define a square line across a board, a line 90 degrees ( a right angle) to the edge of a board.
For more on the history of the Swanson speed square you may watch this video.
Step 1: Identify the Main Features of a Speed Square
LIP- In this picture you'll notice the lip labeled along the left side. If you look at your square, the lip is the bit that is wider than the rest of the square, the part that makes it not sit flat on the table. The lip is what we use to hook on to the edge of a board to know that we have the square where we want it. It gives us our reference point.
PIVOT- In this picture you'll see the pivot in the bottom left-hand corner. The pivot point is the point at which we rotate the square when we are looking to mark an angle other than 90 degrees. It is the basis for reading all angles.
HYPOTENUSE- This is long side of any triangle. On a speed square it is the edge at which we read the degree scale. This is all you need to know about it for now, but I will refer to the hypotenuse later, so I want to make sure you know which edge it is.
Step 2: Using the Degree Scale
Now that you are looking at the correct scale, it's time to find an angle. Take a board, a 2 x 4 or 1 x 4 works fine, and hold the square so the lip is flat against the long edge of the board and the square lies across the face of the board. LIke the picture, only the lip will be completely along the edge of the board. The edge that is square to the lip should run straight across the face of the board forming your 90 degree angle.
Notice where the pivot point is. This is always going to stay on the edge of the board. The edge of the lip that the hypotenuse abuts is going to move. Pivot stays, hypotenuse moves.
Do you see in the picture how the edge of the board forms a ray from the pivot out to the degree scale? With the pivot point of the lip still against the edge of the board, look at where the edge of the board passes through the degree scale. That number is your angle. In this picture, the square is held at 25 degrees.
Marking your board- Now that your square is in the right position you need to draw your line so you know what to cut or how to position your pieces. ALWAYS DRAW YOUR LINE OUT OF THE PIVOT POINT. What this means is that your line should run along the square edge from the lip. 90 degrees from the lip. Along the edge that reads inches. DO NOT DRAW ALONG THE HYPOTENUSE.
Step 3: Using the Common Scale
First we need to make sure that we are looking at the correct scale. The common scale is marked as such, COMMON. It has numbers that range from 1 to 30. The common scale is the next scale in from the hypotenuse.
What is the common scale? The common scale is used to read pitch, for roofs as an example. Roof angles could be identified by degrees but more commonly they are defined by pitch. If you remember back to math class, pitch is defined by rise over run. Run is a certain amount of distance along something. Rise is a certain distance up from something. In construction the run is defined as a constant 12, as in inches in a foot. The rise that occurs over the the 12, gives you your pitch. It does not matter if you are reading in inches, feet, or even (for some reason) centimeters. As long as your run is a multiple of 12, your rise will be in the common scale.
Calculator.net has more information about roof pitches.
In the picture, I held my pivot point on the edge of the board and rotated my hypotenuse down. I read this scale as 7. Which is also 7 in 12, or 7/12, or 7 over 12. These all mean the same thing, For every 12 I go over, I will go 7 up. If it is 12 inches over, I will go 7 inches up. If it is 12 feet over, I will go 7 feet up. If I want my trim board to be cut at the correct pitch to match the roof, I can just use the common scale of 7. That way it doesn't matter how much I go up or over, I am at the same pitch.
REMINDER: Be sure to mark your board along the square edge. The edge that runs away from the pivot point at a 90 degree angle to the lip. This is the most common mistake I see.
Step 4: The Rest of the Speed Square
If you would like to know more about the other uses of the speed square please check back later for more instructables. I have also found several good resources searching that other video tutorial site, you something or other.
I hope this helps. Please ask any questions and feel free to offer suggestions on how to make this more clear.