From Style in Guide part of ABC of Plain Words by Sir E Gowers (1951)

A sentence is not easy to define. Many learned grammarians have tried, and their definitions have been torn in pieces by other learned grammarians. But what most of us understand by a sentence is what the Oxford English Dictionary calls the "popular definition":"such a portion of composition or utterance as extends from one full-stop to another". That definition is good enough for our present purposes, and the question we have to consider is what general guidance can be given to a writer about what he should put between one full-stop and the next.

The two main things to be remembered about sentences by those who want to make their meaning plain is that they should be short and should have unity of thought. Here is a series of 84 words between one full-stop and another, which violates all the canons of a good sentence. In fact it might be said to explode the definition, for it would be flattering to call it a "sentence". "This is not a sentence", said a friend who was good enough to look through this book in proof. "This is gibberish".

Forms are only sent to applicants whose requirements exceed x, and in future, as from tomorrow, forms will only be sent to firms whose requirements exceed 5x, and as you have not indicated what your requirements are, I am not sending you forms at the moment because it is just possible that your requirements may be well within these quantities quoted, in which case you may apply direct to the usual suppliers, of which there are several, with a view to obtaining your requirements.

If we prune this of its verbiage, and split it into three short sentences, a meaning will begin to emerge.

Only firms whose requirements exceed 5x now need forms. Others can apply direct to the suppliers. You do not say what your requirements are, so I will not send you a form unless I hear that you need one.

The following is an even worse example of a meandering stream of words masquerading as a sentence:

Further to your letter of the above date and reference in connection with an allocation of . . ., as already pointed out to you all the allocations for this period have been closed, and I therefore regret that it is not possible to add to the existing allocation which has been made to you and which covers in toto your requirements for this period when originally received, by virtue of the work on which you are engaged, a rather higher percentage has been given to you namely 100 per cent of the original requirements and at this stage I am afraid it is not practicable for you to increase the requirement for the reasons already given.

The fault here is excessive verbiage rather than of combining into one sentence thoughts that ought to have been given several. The thought is simple, and can be conveyed in two sentences, if not in one:

Your original application was granted in full because of the importance of your work. I regret that the amount cannot now be increased, as allocation for this period has been closed.

Now let us look at the other side of the picture, and examine a letter written by someone who had learned the value of short sentences. If the writer had believed in getting as much as possible between his full-stops, it might have run

I enclose herewith the usual forms relating to new claims for sickness benefit which please return after inserting in the blank square after the N.I. number on the forms the suffix to your N.I. number which is printed on your N.I. card after the six figures.

That would be grammatical and intelligible. But it would not be readily intelligible, and the writer of this letter properly determined to make himself readily intelligible. So he made five sentences of it and wrote:

I am sending with this letter the usual forms relating to new claims for sickness benefit. When returning them will you please insert the suffix to your N.I. number. It is printed on your N.I. card after the six figures. You will see that there is a blank square after the N.I. number on the forms. That is where the suffix should go.

This may perhaps be criticised as a trifle too staccato, and it would in fact be improved if the second sentence were joined to the third, and the fourth to the fifth, by colons or semi-colons. But the short sentences do present the reader one by one with the points that he has to take in. The full-stops seem to say to him: "Have you got that? Very well; now I'll tell you the next thing". If the sentence is long he goes on reading it to the end, never quite sure whether a point is complete, and then goes back and reads it again to disentangle the points. (See also Full-Stop.)