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What is the square head used for?

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A black Starrett square head, scriber, spirit level, 90 degree angle, 45 degree angle The square head is arguably the most commonly seen part of the combination square set.
A diagram showing a mitre joint It is traditionally used for determining whether angles are at right angles or 45 degree angles (such as a 45 degree mitre in a mitre joint).
The square head with the 90 degree angle labelled; combination square, rule, ruler, blade, Bahco For this purpose, it has one edge which rests at right angles to the rule…
Square head with the 45 degree angle labelled; combination square, rule, ruler, blade, Bahco …and one which rests at a 45 degree angle to the rule.
Using a square head for marking a piece of wood; rule, ruler, blade It can also be used when marking an object (such as a plank of wood) for cutting…
A square head, depth gauge, rule, ruler, blade …and when measuring depths (e.g. depths of grooves/channels).
Square head, 45 degree angle, combination square, rule, ruler, blade, wood, marking angles As mentioned above, one edge of the head can be used to measure 45 degree angles, but the same edge can be used to measure 135 degree angles as well.
The scriber; combination square set, square head Contained within many square heads is a “scriber” (sometimes referred to by variants such as “scribing tool”), which can be used to make marks on objects.
Sheet metal This is particularly useful when working on metal, where pencils would be useless.
Starrett square head with labelled spirit level; combination square set In addition, square heads normally have a spirit level built in.