In CW reception, we take the meaning of each word. The process goes on with each word as the basic unit. Each character, phrase or sentence won't be the basic unit. We understand the message as composites of words.
It is a common knowledge that we won't be conscious of when reading printed matter as well as copying CW. However, as for transmission, we could forget that rule easily even though we should be careful for that to send the message in decent and easy to copy way.
Let the space between characters as x, that is, the length of a dash in an ideal case, while that between words as X, that is, the length of two dashes or even longer. The message sent on CW could be most easily understood when x<<X. Since the shortest limit of x is the length of a dash as stated above, X should be as long as possible to have this inequality achieved. In other words, compared with x, X should be long enough. When this inequality is achieved, the message is quite understandable.
Even if X is long enough. some operators are inclined to have x elongated. That is, x is even nearly equal to X in the worse cases. It is quite difficult to understand the message word wise. Keep x at proper length in sending while X be long enough. Such a sender with x being nearly equal to X is not rare, unfortunately. They make my brainstormed!
It should be also pointed out that, if you would like to emphasize a certain word, you should have the X before the word longer than usual. It will accentuate the word to good effect.
From the standpoint of reception, any message with this inequality achieved is the most readable one.
This is only a simple fact. But some operators won't be concerned the long X in transmission. Please be careful about it. The you would be praised for your sending.