Theory:

Most of us are born with siblings, big or little, boy or girl. Those born with brothers or sisters in the family know very well that just like sharing, having fun and loving each other's company, fights are also a very important part of sibling life. In fact, any family where sibling rivalry doesn't happen cannot be a normal family because just like variety is the spice of life, fighting with brothers and sisters is the spice of childhood.
 
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Our minds go to this mode whenever we quarrel (Image for illustrative purpose only)
 
As already mentioned in about the poet, Eleanor Farjeon had a childhood where she didn't have a lot of chance to interact with others because of her poor eyesight. She had spent most of her time with books and the only other way to pass the time when she was young would have been to play with her brothers. She describes one such incident where a fight broke out between herself and one of her brothers in "The Quarrel".
 
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Sibling rivalry begins here (Image for illustrative purpose only)
 
In line \(1\), she jumps straight into the story by plainly saying "I quarrelled with my brother" which tells us the situation in a single line, that she and her brother had not agreed on something and that had started a fight between them. Lines \(2\) to \(8\) go on to describe how the little quarrel had escalated quickly into a big fight. For example, in line \(2\), Eleanor says that although the fight had started between her brother and herself, she did not remember the reason behind it.
  
Lines \(3\), \(4\), \(5\) and \(6\) continue telling us how quickly a quarrel can grow in size as she says that one thing led to another and somehow they fell out. The words "One thing led to another" in line \(3\) tells us how quickly a fight can become big and not much attention is given to what had happened, but only on how quickly things are going out of control. Here the words "fell out" in line \(4\) is very interesting because it is not both brother and sister falling out of the door or window hurting themselves, but they "fell out" because both of them thought that what they had thought was right and that the other person was wrong. Lines \(5\) and \(6\) continue to tell us how a strong fight can grow so quickly out of the simplest of reasons, and no real reason is actually needed for a quarrel to happen as the poet already tells us that she had forgotten the reason for the quarrel (line \(2\)).
 
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When disagreements happen, fights break out (Image for illustrative purpose only)
 
In line \(7\), Eleanor tells us that her brother said that he was right, but in line \(8\), she also tells us that she knew that he was wrong. This shows that in that moment of anger Eleanor gives more weight to her feelings than her brother's by using the verb 'knew' showing that she not only said but completely believed that her brother was wrong. This shows how we fiercely defend our side of an argument when we are angry while not at all caring about other people's thoughts on the matter.
 
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When fights break out, fall-outs happen (Image for illustrative purpose only)
  
The fight broke out between the siblings and quickly escalated into a serious quarrel, Eleanor continues the story saying that both brother and sister absolutely hated each other. The sibling love and all their moments of togetherness, sharing, caring, and love had been completely forgotten because of this heated exchange of words. In line \(10\), she goes on to say that the afternoon turned black. This can be taken in two ways. One, the fight between the siblings was so bad that it turned the afternoon, which is the brightest time of the day to darkness. Two, the siblings were so absorbed in their fight that they had not noticed the time passing as the afternoon became dusk. In the first explanation, it highlights the darkness in mood present in the minds of both the siblings and in the second explanation, it shows how a fight can make any person forget about the outside world and stay completely concentrated in being angry upon each other.
 
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Suddenly all those moments of brotherly and sisterly love are lost (Image for illustrative purpose only)
 
There is a sudden shift in the dark mood of the poet, and her brother as in lines \(11\) and \(12\) her brother takes the first step towards both brother and sister becoming caring friendly and loving siblings again by thumping Eleanor on the back. Her brother is unable to hold the grudge of anger anymore and simply tells her the truth that no matter what the disagreement between them, they cannot hold a grudge against each other all night. This shows that even though the fight seemed really big and dangerous, it was only a short amount of time that they could be angry at each other because after all, they are brother and sister and cannot live without one another. These are the feelings of the brother in lines \(13\) and \(14\).
 
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The best siblings are ones who set apart their differences and continue to love each other (Image for illustrative purpose only)
  
Finally, we can see that Eleanor also agrees with her brother to let the silly fight go and in the last two lines (\(15\) &\(16\)) she said that she was wrong to be angry with her brother and since her brother wants this fight to end and their relationship as brother and sister to continue, the brother first agrees that he was wrong and told Eleanor that she was right. The beauty of the last two lines is that in the last line (line \(16\)), she realizes that she had been equally wrong in thinking that she could keep on fighting with her brother. At this moment the entire quarrel seems pointless and its result, whether the brother was right or the sister, didn't seem important anymore, so she immediately drops the fight and both brother and sister become friendly once again.
 
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They were born into the same family after all! (Image for illustrative purpose only)