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*Digital Principles and Applications*by Malvino, Leach and Saha.

Under the heading "Decade Counters", the book lists only Mod-5 and Mod-10 counters. It doesn't state why these are decade counters, or what decade counters are. Others like Mod-8, Mod-3, Mod-6 etc. are listed outside that heading.

All the counters are using JK flip-flop in this book. While I know that other flip-flops can be used to make the counters, I request you to stick to JK flip-flops, because I am new to this topic and am already confused.

For reference, here is the diagram of the Mod-5 up counter given in my book (outputs are A, B and C; C is the MSB, while A is the LSB.):

and here is the Mod-8 up counter (outputs are A, B and C; C is the MSB, while A is the LSB.):

I searched Wikipedia to find out what decade counters are.

This makes things confusing for me. How can something count in decimals? Consider the Mod-5 counter above. It goes from 000 to 100. Isn't that counting in binary rather than decimal?Wikipedia said:A decade counter is one that counts in decimal digits, rather than binary.

Speaking the other way round, the Mod-8 counter goes from decimal 0 to 7. I can easily get the output in decimal if I decode the necessary outputs. So why is it not a decade counter?

https://www.electronics-tutorials.ws/counter/mod-counters.html says,

According to this definition, a Mod-5 counter doesn't qualify as a decade counter because it has five states, from decimal 0 to 4.Decade (divide-by-10) counters such as the TTL 74LS90, have 10 states in its counting sequence making it suitable for human interfacing where a digital display is required.

So, why are Mod-10 and Mod-5 decade counters and Mod-6 and Mod-8 not? Or is the book incorrect? Or am I misinterpreting something?