Mitre Box
From DT Online
Mitre Boxes (aka Mitre Blocks) are used to guide saws to cut accurately 45 degree mitres to make corner joints - and also 90 degree right angle cuts.
They are typically used with a Tenon saw, but for modelling work and craftwork, smaller versions are available for use with a Razor Saw or Junior Hacksaw.
Mitres are also easily and safely cut using a Precision Mitre saw and trained adults can use a powered Mitre saw.