The form of writing in Devanagari did change over the centuries, but the base of the script was laid on firm foundations that all scripts of India have inherited from their ancient mother script.
The characteristics of Devanagari are as follows:
Devanagari Needs No Spelling Arrangement:
Spelling has been eliminated in Devanagari because every sound had been correctly analyzed and placed into its phonetic classification and the consonants and the vowels which have different functions have been assigned definite mode of behavior. On the contrary as combination of letters are used in Roman script for sound which have no signs, the need for spelling them artificially became necessary.
Series Modulation by Vowel Signs:
Devanagari and all script of the Brahmi family have distinctive graphemes for all vowels in order to join with the consonants representative signs. These have been evolved along with the script.
Differentiation for the Pronunciation of Vowels:
As pronunciation was to be very accurately managed the Indian Grammarians made differentiation between the signs for the short and long sounds of the same vowel. It will be seen that the vowels which are short, flourish to the left and their longer signs to the right. This is noticeable in Devanagari, Tamil, Malayalam and other scripts of South Indians who were very careful in preserving their traditions.
The Vertical Line:
In earlier Brahmi script the vertical line is absent; it shows the addition of the Aa matra to a consonant so that it could be fully pronounced and written.
In some letters the vertical line is short and goes on the top of forms, because a full line cannot be drawn through. The vertical line is an essential part of any Devanagari letter and in this context the reference in Bhagavad Gita is remarkable. The vertical line is cancelled by a halant sign in the case of letters to represent the half pronounced phoneme. These half sign also help to form conjunct letters. The suggestion to make half letters from full letters by adding halant signs has been found helpful for pronunciation.
The Top Line:
The top line is an integral &essential feature of Devanagari orthography. The continuous top line is a later development. In old manuscripts, copper plates and inscriptions the top line was limited to the character only and did not join the next letter. The top line knits letters into distinct words. A gap indicates separation of one word from another.
Conjunct Formation of Consonants:
Devanagari has evolved as a specialized script for the highly developed languages like Sanskrit. Some reformers recommended formation of conjunct letters with halant sign even in case of letters with full vertical lines. For saving space the conjuncts in Devanagari were initially written vertically, but the Grammarians would not object to joining then lineally when half forms of letters with full vertical line could be used by removing the vertical line. In the case of archaic letters there is no other way than applying the halant sign especially in case of composing with a machine like typewriter.
Diacritical and Special Marks:
Devanagari script is said to be capable of expressing many varieties of sounds. Various signs for variety of long and short vowels were made in the language. But since the signs vary from province to province, an effort in standardization is very essential. Affricated words which came to India through the contact of the Persians and the Arabs left a mark on Hindi and such sounds have been denoted in Devanagari by adding a point at the bottom nearer graphemes in Devanagari.
There also exist another set of vowels which have been added to expand the range of traditional Devanagari. The European introduced some sounds which are pronounced with half open articulation e.g. cap, gap top, all etc. and these are denoted in Devanagari by putting half-moon marks on the top of nearer grapheme.
Direction from Left to Right:
Perhaps all ancient scripts were first written from the right to left. Only the Hebrew, the Arabic, the Persian and the Urdu continued with old way, but Brahmi the mother script of Devanagari changed the direction of writing from left to right. In this respect the Devanagari is somewhat nearer to the Roman script.
Independent Grapheme exists for Pure Vowels and Consonants:
Like the Roman script, Devanagari too has independent vowels. Every basic vowel has a different phonetic origin; each should reasonably have independent grapheme. These vowels are given representative forms and these are used for modulation of consonants which are supposed be pronounced with the help of vowels into a modulating series.
Basic Arrangement of Phonemes is Phonetic:
The Brahmi script has evolved from the studies in phonemics in which the Dravidians had specialized and hence is completely phonetic. There are some pronunciations imported from foreign languages and they are indicated by a dot at the bottom of the nearer phoneme. Half open vowels imported from European tongues are indicated by a half-moon sign over nearer phoneme.