In Secret Transmissions, the check digits were in position 1, 2 and 4 of the 7 digits, and the message digits were in position 3, 5, 6 and 7.

Digits 1, 3, 5 and 7 contained an even number of 1s.

Digits 2, 3, 6 and 7 contained an even number of 1s.

Digits 4, 5, 6 and 7 contained an even number of 1s.

What do you notice about the position of the check digits in the message?
Where might you put the next check digit in a longer message?
How can you determine which message digits 'belong' to each check digit?