Kimariji (a term used for Hyakunin Isshu play cards) (決まり字)
Kimariji is a term used for Hyakunin Isshu play cards (a game based on a famous poetry anthology, One Hundred Poems by One Hundred Poets); when a card of the first half of a poem is read out, players compete to take a matching card of the last half, and Kimariji means the first several letters of poems with which players can be sure which card to take. Particularly in Karuta (card) competitions that require the speed of taking cards, it is vital to understand Kimariji.
Summary
For example, Ogura Hyakunin Isshu (the Ogura Anthology of One Hundred Tanka-poems by One Hundred Poets) used for Karuta competitions includes three poems that start with the letter "い" (i) as follows.
いにしへの (inishieno)
いまはただ (imawatada)
いまこむと (imakomuto)
Then Kimariji are "いに" (ini), "いまは" (imawa) and "いまこ" (imako) respectively.
However, as the game goes on and many cards have been taken away, these Kimariji will change.
If the card of "いまこ" (imako) has been taken away, then Kimariji of "いまは" (imawa) changes to "いま" (ima). Further, if the card of "いに" (ini) has also been taken away, then Kimariji of "いま" (ima) changes to "い" (i).
In addition, Karuta competitions have a rule that touching a wrong card is not considered as a mistake if that card is on the same territory as the right card to be taken. Thus, if all the three cards of "いに" (ini), "いまは" (imawa) and "いまこ" (imako) are on your territory or the opponent's territory, then all the cards can be taken with the letter "い" (i).
Similarly if, for example, "きみがためは" (kimigatamewa) and "きみがためお" (kimigatameo) are on the same territory, then both can be taken with the letters "きみ" (kimi).