The saw guide slots on a mitre box are to guide the saw blade, preventing the saw from wandering across the workpiece and ensuring the correct angle is cut.
45 degree saw guide slots
All mitre boxes come with two 45 degree saw guide slots.
By placing your workpiece, for example a piece of skirting, into the mitre box and cutting it using one of the 45 degree saw guide slots and then placing another piece of skirting into the mitre box and then cutting the skirting using the other 45 degree saw guide slot you will achieve a precise 90 degree corner when the two lengths of skirting are placed together.
Side-on 45 degree saw guide slots
Most of the general purpose mitre boxes come with a 45 degree side-on cutting facility.
This feature allows the user to place the workpiece into the bottom of the mitre box to achieve a precise 45 degree cut when the width of the workpiece would be taller than the side walls of the mitre box, and consequently the saw blade would not be able to utilise the saw guide slots.
90 degree saw guide slots
The 90 degree saw guide slots are designed for producing a precise straight cut on your workpiece.
When a precise and square 90 degree cut is required, for example when joining plastic piping together, the 90 degree saw guide slots on a mitre box are the ideal way of achieving this.
22.5 degree saw guide slots
Some mitre boxes come with 22.5 degree saw guide slots, enabling the user to create perfect 45 degree joints.
By placing your workpiece, e.g. a dado rail, into the mitre box and cutting it using one of the 22.5 degree saw guide slots and then placing another piece of dado rail into the mitre box and cutting the dado rail using the other 22.5 degree saw guide slot, you will achieve a precise 45 degree corner when the two lengths are joined together.