What is Al Hamichya?

Al Hamichya is the bracha acharona (after-blessing) said after eating foods made from the five grains in mezonos form, such as cake or crackers, rather than bread.

When do you say Al Hamichya?

It is said after eating at least a kezayis of mezonos grain foods, as well as after certain wine and the special fruits of Eretz Yisrael. Confirm the details with your rav.

Why keep an Al Hamichya card or sign on the table?

Because it is said from memory less often than Birkas Hamazon, a card or sign keeps the correct text at hand for everyone at the meal.