I remember a typing teacher being very serious about two spaces after the dot[1]. He was also serious about the letter O being an acceptable substitute for a zero "0". Fortunately both of these rules are now firmly in the past along with witch burning. This is progress. [1] yeah, I could call it a period or a full stop. But its also a dot. Variety is the spice of life as is properly finishing your
Type-set lead blocks allowed for variable spacing, typewriters were more limited. When I see double-spaces now (in email, for example) I tend to draw conclusions about the sender's age.