On what grounds are the Vedas divided ? There are many suktas and mantras in two or more Vedas which are chanted differently. I am a novice, so I apologise if I have transgressed some rules while asking this question.
In his General Introduction to his translation of the Upanishads, Swami Nikhilananda states (The Upanishads, Vol 1, pp 2-3):
The Division Of The Vedas
...In the Puranas it is stated that Vyasa was commanded by Brahma to make a compilation of the Vedas. Vyasa is reputed to be the author of the Mahabharata, of which the Bhagavad Gita forms a part. He lived at the time of the battle of Kurukshetra. With the help of four disciples, so the tradition goes, this great saint and poet arranged the Vedas into four books, namely the Rik, Yajur, Saman, and Atharva. He was thus the classifier of the Vedas, though not their author. For many centuries before his time the vedas had been known and their injunctions had formed the basis of all Hindu philosophic thought and all brahminical ceremonial. But the texts had not existed in a systemic form. THey had been revealed by the Lord to certain holy men of the remote past who had purified their minds by the practice of such spiritual disciplines as self-control and concentration--the great teachers known as the rishis, or seers of truth. The Vedas name both men and women among the rishis.
Vyasa compiled the Rig-Veda by collecting the riks. Of the samans he composed the Sama-Veda, while the Yajur-Veda he composed of the yajus. The Rig-Veda which may be called the book of chants, is set to certain fixed melodies. The Sama-Veda has no independent value; for it consists mostly of stanzas from the Rig-Veda. The arrangement of its verses os solely with reference to their place and use in the Soma-sacrifice. The Yajur-Veda contains, in addition to verses taken from the Rig-Veda, many original prose formulas which may be called sacrificial prayers. The Atharva-Veda consists of a special class of Vedic texts known as chhandas. These deal with spells, incantations, and kingly duties, as well as exalted spiritual truths. Western scholars sometimes exclude this compilation from their consideration of the Vedas; but according to the Hindu view it definitely belongs among them. The name Trayi, or Triad, often used to denote the Vedas, is collectively applied to the RIg-Veda, the Sama-Veda, and the Yajur-Veda, the Atharva Veda being excluded from the Triad because it has no application to sacrificial actions. Nevertheless, one of the four priests officiating in all Vedic sacrifices had to be thoroughly versed in the chhandas.
The word Veda means wisdom, knowledge or vision, and it serves to manifest the language of the gods in human speech. The laws of the Vedas have regulated the social, legal, domestic and religious customs of Hindus up to the present day. All the obligatory duties of Hindus at birth, marriage, death etc. are guided by Vedic rituals.
ORIGIN OF THE VEDAS It is difficult to say when the earliest portions of the Vedas came into existence, but it seems clear they are among the very earliest written wisdom docments produced humans. As the ancient Hindus seldom kept any historical record of their religious, literary and political realization, it is difficult to determine the period of the Vedas with precision. Historians provide us many guesses but none are guaranteed to be precise. It is thought, though, that the earliest Vegas may date back to roughly 1700 BCE—the late Bronze Age.
WHO WROTE THE VEDAS? Tradition has it that humans did not compose the revered compositions of the Vedas, but that God taught the Vedic hymns to the sages, who then handed them down through generations by word of mouth.
Another tradition suggests that the hymns were "revealed" to the sages, who were known as the seers or “mantradrasta” of the hymns. The formal documentation of Vedas was done mainly by Vyasa Krishna Dwaipayana around the time of Lord Krishna (c. 1500 BC)
CLASSIFICATION OF THE VEDAS The Vedas are classified into four volumes: the Rig-Veda, the Sama Veda, the Yajur Veda and the Atharva Veda, with the Rig Veda serving as the principle text.
The four Vedas are collectively known as “Chathurveda, ” of which the first three Vedas--Rig Veda, Sama Veda and Yajur Veda--agree with one another in form, language and content.
STRUCTURE OF THE VEDAS Each Veda consists of four parts--the Samhitas (hymns), the Brahmanas (rituals), the Aranyakas (theologies) and the Upanishads (philosophies). The collection of mantras or hymns is called the Samhita.
The Brahmanas are ritualistic texts that include precepts and religious duties. Each Veda has several Brahmanas attached to it.
The Aryanyakas (forest texts) intend to serve as objects of meditation for ascetics who live in forests and deal with mysticism and symbolism.
The Upanishads form the concluding portions of the Veda and is therefore called the “Vedanta” or the end of the Veda. The Upanishads contain the essence of Vedic teachings.
THE MOTHER OF ALL SCRIPTURES Although the Vedas are seldom read or understood today, even by the devout, they no doubt form the bedrock of the universal religion or “Sanatana Dharma” that all Hindus follow. The Upanishads, however, are read by serious students of religious tradition and spirituality in all cultures and are regarded as principle texts within the body of mankind's wisdom traditions.
The Vedas have guided our religious direction for ages and will continue to do so for generations to come. And they will forever remain the most comprehensive and universal of all ancient Hindu scriptures.
Next, let's look at the four Vedas individually,
“The One Truth the sages call by many names.” ~ Rig Veda
THE RIG VEDA: THE BOOK OF MANTRA The Rig Veda is a collection of inspired songs or hymns and is a main source of information on the Rig Vedic civilization. It is the oldest book in any Indo-European language and contains the earliest form of all Sanskrit mantras, dating back to 1500 BCE- 1000 BCE. Some scholars date the Rig Veda as early as 12000 BCE - 4000 BCE.
The Rig-Vedic ‘samhita’ or collection of mantras consists of 1,017 hymns or ‘suktas’, covering about 10,600 stanzas, divided into eight ‘astakas,’ each having eight ‘adhayayas’ or chapters, which are sub-divided into various groups. The hymns are the work of many authors, or seers, called ‘rishis.’ There are seven primary seers identified: Atri, Kanwa, Vashistha, Vishwamitra, Jamadagni, Gotama and Bharadwaja. The rig Veda accounts in detail the social, religious, political and economic backgro