Is it grammatically incorrect to put only one space between sentences?
This is not a grammar question. It is a typography question. When using typewriters which have the same width for every character and space, two spaces is conventional. When using proportional fonts one space is the rule. A well designed font will have the right amount of white space after periods and colons and other punctuation to make the text look right with just one space. Monospaced devices like typewriters had special rules for segregating sentences and other situations. When the Macintosh computer was introduced, bringing proportional typography to regular people, a book on typography was written to help understand the differences between regular typewriters and proportional typesetting: “The Mac is not a Typewriter” I still have it on my shelf.
