Monko-fuda are cards used in Kodo (traditional incense-smelling ceremony).
In kodo, sniffing of the incense is expressed as 'listening' to the incense, and as such, it is called monko (in kanji, 聞香 (lit. listening to the incense)). The term "monko" implies tasting the scent within your mind, and not simply sniffing the scent. One enjoys the scent of incense rising from the koro (incense burner) held inside their palms. Games were also employed, in which people guessed the type of the incense being burnt.
The 'Yugi-ko' (A Review of Play) part of the "Kojiruien" (Dictionary of Historical Terms), an encyclopedia published in the Meiji period, introduced the rules of the game as follows:
A participant first listens to three types of incense which is given a number from 1 to 3; then, afterwards, a certain burning incense is brought before the participant. If the participant guesses that the burning incense is the same as incense #1, he or she puts the #1 monko-fuda into a cylinder. Similarly, if participant guesses it to be the #2 incense, he or she puts the #2 monko-fuda into the cylinder, and if participant guesses it to be the #3 incense, he or she puts the #3 monko-fuda into the cylinder. If the participant guesses that the burning incense is not among the three incenses listened to beforehand, he or she puts the "客" (lit. a guest) monko-fuda into the cylinder. The participant who is able to guess these incenses more correctly than others becomes the winner.
Monko-fuda's were excavated from the ruins of Kusado-sengen City (presently, Fukuyama City) and the castle ruins of Motoharu KIKKAWA Yakata (Kita-hiroshima-cho) both in the Hiroshima Prefecture, and from the site of Akahori-jo Castle (Yokkaichi City) in the Mie Prefecture. The monko-fuda's excavated from the site of Akahori-jo Castle have the kanji's '三' (lit. three) and '嶋' (lit. island) written on them in Indian ink.