Since DUNGEONS & DRAGONS was originally written for wargamers who are used to miniature figures, distances are often given in inches. Inches can be converted to feet by multiplying b ten: 1 inch = 10 feet, 2 inches = 20 feet, etc. This scales movement appropriately for maneuvering the figures on the top of a gaming table.It's an interesting aside for several reasons, the foremost being that I like how Holmes makes an effort explain the origin of using inches for movement and scale in D&D. As a kid, I was initially quite baffled by this presentation in the Monster Manual and (later) the Players Handbook. I'm not sure that AD&D ever explains it; instead it simply assumes that readers will know the convention. I'm pretty sure I learned it from one of the older guys I knew, perhaps even my friend Mike's teenage brother.
The second reason why the aside is interesting is that the Holmes rulebook doesn't use inches in its text by and large. Both spells and monster movement rates are given in feet, in contrast to OD&D and AD&D, which retain the inches convention. Personally, I don't have any problem with using inches, since it's now second nature to me to translate between inches and feet when reading old school D&D books, but I nevertheless thought it notable that, even back in 1977, Holmes thought it worth explaining the usage and where it came from.