(a) Phonetics, which teaches us to recognize and to reproduce sounds and tones.
(b) Orthography, which teaches us to spell what we have already learnt by ear.
(c) Accidence and etymology, which teaches us the nature of inflected forms and derivatives, and also how to use them.
(d) Syntax and analysis, which teaches us how to build up sentences from their components.
(e) Semantics, which teaches us the meanings of words and forms.
We observe the principle of proportion when we pay the right amount of attention to each of these five branches, without exaggerating the importance of any of them.
In choosing the units of our vocabulary we may be guided by several considerations, such as intrinsic utility, sentence-forming utility, grammatical function, regularity, facility, concreteness, or completeness. We observe the principle of proportion when we select the material of our vocabularies in such a way that due attention is paid to all such desiderata, and without exaggerating the importance of any of them.
We also observe the principle of proportion when we give the right amount of drill-work or free work, of translation-work or ‘direct’ work, of intensive reading or extensive reading. A well-proportioned course, like a well-graded course, ensures a steady and ever-increasing rate of progress.