of the code
makes it easy to tell the difference between letters, until you get into the very high (30-40 words per minute) range
To make it easier, many folks use something called Farnsworth Code or Farnsworth Weighting. If you were sending a message to someone that understood morse at about 5 words per minute (WPM), you could send the dits and dahs at a higher rate but left more blank time between the letters. The white space between the letters allows the receiver to translate the morse.
Most morse code tests in the US use Farnsworth Weighting.
The United States FCC decided to go along with the rest of the world and lower the code test for amateur radio operators to five words per minute. With newer digital signaling technologies, morse is going by the wayside.