Can you never remember to feed the hamster? Or do you always forget to record your favourite show?
An antiquated form of memory aid. The basic idea is that you tie a knot in the corner of your handkerchief the night before you have to perform some task. When you go to blow your nose or wipe your face or perform any of the other actions associated with a handkerchief you will see the knot and remember your duty.
Problems arise, however, when it comes to recalling precisely what it is you are supposed to remember. Also, with the rise of the paper tissue, and the decline of the handkerchief, this practice has fallen out of use. Have you ever tried to tie a knot in a paper tissue? It is very difficult, it becomes ragged and torn, you will soon become frustrated, and may swear profusely. Nowadays it is far easier to set an alarm on your mobile phone.