On page 182 of the PHB, there is a section titled Climbing, Swimming, and Crawling that contains no details about crawling. In the section, it says:
While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed.
Does this apply to crawling speed as well?
This is answered on page 191 of the PHB:
Being Prone
[...] To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.
The rules for crawling are on page 191. Which is to say, in the chapter on Combat, under the Being Prone section.
To move while prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot. Crawling 1 foot in difficult terrain, therefore, costs 3 feet of movement.
Why the section you're looking at includes Crawling in the title, I don't know.
The rules for it are on page 191, under the "Being Prone"section:
To move while Prone, you must crawl or use magic such as teleportation. Every foot of movement while crawling costs 1 extra foot
