Napier's bones are an easy way of multiplying large numbers without losing track of all the columns, rows, carrying...
The original version (repeated in this instructable
) consisted of sticks (bones) with numbers marked on them, but that's not so portable.
The process can, though, be repeated with pencil and paper.
In school, this method is suitable for classes of most ages who are getting to grips with multiplying larger numbers.
In the UK, KS2 and upwards.