Then there's my favorite (not counting Church numerals), biquinary, in which decimal digits are represented by exactly two bits on, out of five (so there are 5×4/2=10 possible choices).The bits have numeric values 0, 1, 2, 4, and 7; two of these will add up to any digit 1-9, but the digit 0 is exceptionally represented as 4+7. (Wikipedia uses the name "biquinary" for a different, seven-bit code, but I was taught that it meant the code described in the first article.)

(The point of this strange system is that it has some redundancy in it -- four bits are more than enough for decimal digits -- and in the old days computer memory was prone to failure.)

I'm moving to the account @warpedwartwars, as can be seen in the post I just posted here on that account. (said post needs approval because I made the account about an hour or two ago. (except now it doesn't because it has been approved.))