MSPS are not prevented from taking on extra work to supplement their basic salary of £58,678.

Their earnings are listed in a register of members' interests published on the parliament's website.

The rules, designed to ensure transparency, are set out in a handy cut-out-and-keep diagram for MSPs and their staff.

If work attracts payment beyond simple expenses, MSPs must submit a written statement to officials.

MSPs must also declare expenses, including 'benefits in kind' such as hospitality, if they amount to more than one per cent of their salary.

Strict rules are in place preventing MSPs from engaging in "paid advocacy," the championing of causes on behalf of an individual or organisations directly in return for cash.

The ban covers "advocating or initiating a cause".

If an MSP, or member of the family, has benefited from an oil company, for example, it does not mean they face a blanket ban on discussing the oil industry in general terms in debates or committee meetings.

In that instance, however, they must declare an interest when they speak.