The combination square consist of adjustable 300mm ruler fitted onto a a 45°/90° square. The square also has a built-in spirit level. it is mostly used in the woodworking, stone masonry and metalworking industries.
 
Fundamentally it is used for:
Verifying 45° and 90° angles. It can be used to check mitre joint accuracy.
Determining flatness. To assist with designating a reference surface when working with wood.
The spirit level assists with installations (kitchens, built-in cupboards etc.)
By adjusting the ruler, the square can also be used as a depth gauge or a marking gauge.