These are machine shop tools that offer different methods of holding a work piece at a desire angle.
Professional shops use standard sized 5 inch and 3 inch sine bars to set precise angles. (In order to use sine bars you need to have gage blocks, see instructions tab for how to use.)
The angle block set is a quick alternative method of setting an angle.The triangle blocks have the angles 5 degrees, 10 deg, and 30 deg which can be stacked to give you any angle between 5-45 deg, in 5 deg increments. (Place blocks in reverse to subtract angles). The J shaped holder keeps them from sliding off of each other.
Steel Sine bar on amazon for reference: http://amzn.to/28OzfdX
To set a really precise angle with a sine bar and stack of blocks, take the sine of the desired angle on your calculator and multiply the result by the distance between the centers of the cylinders in the sine bar. (EIther 5" or 3".) Assemble a stack of blocks equal to this value and put it under one of the cylinders.
If you have a digital angle indicator you can set one end of the sine bar onto a machinist jack and adjust it until you have the exact desired angle. http://www.thingiverse.com/thing:593680
The holes in the sine bar are sized for 5/16" bolts and spaced evenly so they can be mounted to the 1-2-3 block set here: http://www.thingiverse.com/thing:584470