Two spaces is a sensible convention for distinguishing inter-word spaces from inter-sentence spaces. If it's being typeset then the spacing can be adjusted the

Two spaces is a sensible convention for distinguishing inter-word spaces from inter-sentence spaces. If it's being typeset then the spacing can be adjusted then. It's also not true that this is just a convention left over from typewriters. Typeset books frequently have more space between sentences than between words, particularly when the text is justified.