Letters ~ small
To qualify as a Royal Mail Letter the maximum size is limited to 5mm x 165mm x 210mm.
To qualify as a Royal Mail Letter the maximum weight is limited to 100gm.
This allows for any size envelope up to C6, and for a paper envelope containing two or three sheets of folded notepaper. Additional items, such as buttons, are likely to be too thick to pass through the 5mm slot, which is smaller then one quarter inch. In our experience it is the size of an object, rather than its weight, that takes it beyond the Letter category. The next size up is the Large Letter.
Note: we have added 'small' to the word 'Letter' to emphasise the difference from 'Large Letter'. The title 'Small Letter' is not officially used.