It is part of a Whip's job to see that members are kept
informed about forthcoming business - and make sure
they know which issues matter most to the party. Every
week they send a notice (known as The Whip) to MPs,
which gives the order of business for the following week.
In The Whip, each matter to be discussed in the house
will be underlined once, twice, or three times. A single
underline means the matter is not particularly important - and the member's attendance is 'requested'. Two underlines means attendance is 'particularly requested'. But attendance is 'essential' if there are three underlines (a 'three-line whip').
