Why are slope and y-intercept m and b?

[Cunctator]

@Dervorin

In the UK, Australia, Bahamas, Brunei, Germany, Ghana, India, Indonesia, Ireland, Jamaica, Malaysia, Malta, Nepal, New Zealand, Pakistan, Singapore, Solomon Islands, South Africa, Zambia and Zimbabwe
y = mx + c

It was y = mx + b at my school in Australia in the 1970s.