What does the word द्विज[dwij] mean? You might be tempted to say that द्विज means the same thing as ब्राह्मण. But you are not entirely right. Etymologically, द्विज means someone, or something, that is born twice. Hence, a bird, or any other creature that is born from an egg, is called द्विज. Similarly, each of the first three varnas, brahmana, kshatriya and vaishya, is द्विज. There is the physical act of birth and later, there is a second birth at the time of the sacred thread ceremony. The first three varnas were all entitled to the sacred thread ceremony. You may have heard that brahmanas alone were entitled to study the Vedas. In this version of tidbits, we are on Chapter 10 of Manu Smriti. 10.1 of Manu Smriti is very clear. “While engaged in their own tasks, each of the first three varnas, the dvijas, will study (meaning the Vedas). However, only brahmanas are entitled to teach it. That is the rule.” There is a difference between studying the Vedas and teaching them. 10.4 adds to this. “The three varnas, brahmanas, kshatriyas and vaishyas, are dvijas. The fourth, shudras, have only one birth. (That is, they aren’t entitled to the sacred thread ceremony.) There is no fifth (varna).”
Chapter 10 then has a lot of what can be called taxonomy. How do you classify and name those of mixed parentage? But before that, 10.5 tells us that ideally, father and mother should be of the same varna. You may have heard of the expression अनुलोम. This can cause confusion. It is often applied to children of mixed parentage. But it is also applied in the contrary sense of natural or regular birth, meaning that father and mother are of the same varna. 10.5 of Manu Smriti uses the word अनुलोम in that natural sense, of father and mother being of the same varna. Let me now give you that taxonomy of names. Since there are several, it can be quite confusing, especially since these words aren’t always used in the way Chapter 10 of Manu Smriti does.
Brahmana father and vaishya mother produce अम्बोष्ठ, brahmana father and shudra mother produce निषाद or पारशव (10.8). Kshatriya father and shudra mother produce उग्र(10.9). Kshatriya father and brahmana father produce सूत, vaishya father and kshatriya mother produce मागध, vaishya father and brahmana mother produce वैदेह (10.11). Shudra father and vaishya mother produce अयोगव, shudra father and kshatriya mother produce क्षत्ता and shudra father and brahmana mother produce चण्डाल (10.12). Brahmana father and ugra mother produce अवृत, brahmana father and ambashtha mother produce आभीर, brahmana father and ayogava mother produce धिग्वण (10.15). Nishada father and shudra mother produce पुक्कस, shudra father and nishada mother produce कुक्कुटक (10.18). Kshatta father and ugra mother produce श्वपाक, vaideha father and ambashtha mother produce वेण (10.19). Perhaps I should tell you what व्रात्य is too. Vratya isn’t outcast. It is outcaste. Vratya is offspring born when the father and the mother have the same varna. However, because the vratya offspring has deviated from prescribed vows, it is no longer entitled to the savitri mantra (10.20). And so it goes on, with finer and finer classifications. You get the idea. But let me repeat the caution mentioned earlier. These are definitions as per Chapter 10 of Manu Smriti. In other contexts, these words do have different meanings. For several of these groups, Chapter 10 also indicates prescribed occupations.
A brahmana can perform six acts – teaching, studying, performing sacrifices for himself, performing sacrifices for others, giving donations and receiving donations (10.75). Of these six acts, three are forbidden for kshatriyas – teaching, performing sacrifices for others and receiving donations (10.77). These are also forbidden for vaishyas. Bearing weapons is a means of sustenance for kshatriyas and trade, animal husbandry and agriculture for vaishyas (10.79). However, kshatriyas and vaishyas must also donate, study the Vedas and perform sacrifices. If these prescribed modes do not work for a brahmana, he can adopt a kshatriya’s means of subsistence. If those prescribed modes do not work for a brahmana or a kshatriya, he can adopt a vaishya’s mode of subsistence. However, let’s go through the vaishya’s mode of subsistence again – trade, animal husbandry and agriculture. 10.83 tells us that a brahmana or a kshatriya who resort to a vaishya’s mode of subsistence, must not use agriculture. Agriculture is violent (it is injurious to living beings) and depends on others (because those crops have to be sold?) In general, 10.84 tells us that while some people think that agriculture is good, others say that a wooden plough with an iron head is damaging, because it injures the earth and injures living beings who dwell within the ground. There are several commodities a brahmana should not sell (10.94). Examples are flesh, salt and milk. 10.99 also deserves a mention. We generally know that shudras were supposed to earn a living by serving the other three varnas. 10.99 tells us that if this does not work, a shudra can resort to कारूककर्म. This means to work as an artisan and earn a living through handicrafts.
But if a brahmana is hungry and liable to die because of the hunger, deviations are acceptable. For example, Ajigarta was so hungry that he was about to slaughter and kill his own son (10.105). Vamadeva and Vishvamitra were about to eat the flesh of dogs (10.106-108). In such times of catastrophe, deviations from the rules are allowed.
I should have said this right at the beginning. Chapter 10 is relatively short. It only has 131 shlokas. You have formed a general idea of what it is about and this chapter is much less controversial than several other parts of Manu Smriti.