Hmm, I have not seen nor used that L-shaped thing for a long, long time. When I was in my LCE year in 1970, we used that quite a bit in our Industrial Arts class. What is that ?
For folks who still are not able to get it right, it is called a Try Square. We used it to mark out lines on a piece of wood or board, so that the line would be at Right Angles to a particular edge of the wooden piece.
After that a hand saw was used to cut along the marked line. Skill, determination and practice were needed to get it right.
These days, with fewer and fewer people keen on D-I-Y work, preferring swiping touchscreens with their finger tips to flexing their under-used muscles, these ‘vintage’ tools are getting rarer by the day.