[<< wikibooks] Traditional Abacus and Bead Arithmetic/Multiplication
== How many multiplication methods are there? ==
Let's take an example: 
  
    
      
        345
        ×
        6789
        =
        2342205
      
    
    {\displaystyle 345\times 6789=2342205}
  . We do this multiplication by adding the 12 partial products that result from the expansion:

That is, all the products listed in this table:

But these products can be added in any of the 
  
    
      
        12
        !
      
    
    {\textstyle 12!}
   (12 factorial) 
  
    
      
        =
        1
        ×
        2
        ×
        3
        ×
        ⋯
        ×
        12
        =
        479
        
        001
        
        600
      
    
    {\textstyle =1\times 2\times 3\times \cdots \times 12=479\,001\,600}
   ways of ordering them, so we could say that there are, at least, almost 500 million ways to calculate the product of the two given numbers.
But it is clear that, of this immense number of ways of sequentially adding partial products, only a few can be efficiently and safely generated and followed by the human brain. But these few are still a lot ... especially if we think that we can also choose whether or not to enter multiplicand and multiplier in the abacus and where to start adding the partial products with respect to said operands. In what follows we will focus on this last aspect.


== Inverse operations ==
Addition and subtraction are inverse operations in the sense that each undoes the effect of the other by reverting the result to the first operand; for example: 
  
    
      
        422
        +
        313
        =
        735
      
    
    {\displaystyle 422+313=735}
   and now subtracting 
  
    
      
        735
        −
        313
        =
        422
      
    
    {\displaystyle 735-313=422}
  . On the abacus:

and, as we can see, we not only obtain the starting value but also obtain it in the original position. In turn, multiplication and division are also inverse operations; i.e: if 
  
    
      
        a
        
          /
        
        b
        =
        q
        ,
        r
      
    
    {\displaystyle a/b=q,r}
  
where 
  
    
      
        q
      
    
    {\displaystyle q}
   is the quotient of dividing 
  
    
      
        a
      
    
    {\displaystyle a}
   by 
  
    
      
        b
      
    
    {\displaystyle b}
   and 
  
    
      
        r
      
    
    {\displaystyle r}
   is the remainder, we can reverse the operation in the form:

  
    
      
        a
        =
        q
        ×
        b
        +
        r
      
    
    {\displaystyle a=q\times b+r}
  
for example: 
  
    
      
        4727
        
          /
        
        72
        =
        65
        ,
        47
      
    
    {\displaystyle 4727/72=65,47}
   where 65 is the quotient and 47 the remainder and we can reverse the operation in the form 
  
    
      
        4727
        =
        65
        ×
        72
        +
        47
      
    
    {\displaystyle 4727=65\times 72+47}
  . On the abacus, using modern division and multiplication methods:

We have reversed the operation and returned the abacus to its original state. Note the relative position of operands and results using the modern method:

Now let's try the same with the traditional method of division (TD) and the traditional division arrangement (TDA).

now the relative position of operands and results using the traditional method is different:

If we want to reverse the operation by multiplication, we could first proceed by memorizing the digit of the multiplicand to use and clearing it, then we would proceed to add the partial products:

and we have also reversed the operation and returned the abacus to its original state. In this way we proceed exactly the same as with modern multiplication, previously freeing up and reusing the space occupied by the digit in use of the multiplicand. However, memorizing and keeping something in memory while working with the abacus opens a door for errors and it is desirable to minimize this possibility by trying to keep the digit in memory for as little time as possible. This is achieved by altering the order in which we add the partial products:

As we can see, we have delayed clearing the digit in use until the last possible moment. This is the basis of the traditional method of multiplication.


== Traditional multiplication method ==
The traditional method of multiplication was first introduced using counting rods and the best way to introduce it to the modern abacist is to consider that a multi-digit multiplier consists of a head (the first digit from the left) and a body (the rest of the digits); for example: 4567✕23, considering 4567 as the multiplier, its head is 4 and the body 567. So, for each digit of the multiplicand (from right to left):

proceed as in modern multiplication with the product of the digit of the multiplicand by the body of the multiplier
then clear the digit of the current multiplicand and add its product by the head of the multiplier to the column just cleared and the one adjacent to its rightnote: ^a Result is 10F041 if you use the 5th lower bead,105041 otherwise.
But things are not always as simple as in the previous example; if both the multiplicand and the multiplier contain high digits (7, 8, 9) we may have overflow problems and need to deal with them (see chapter: Dealing with overflow), as in the case 999✕999=998001:

The most convenient, as in the case of division, is to have additional upper beads, that is, a 5+2 type abacus or a 5+3 if we are lucky enough. For the 4+1 and 5+1 abaci, it may be best to use the following fallback to the method outlined in the previous section, clearing the current digit of the multiplicand at the beginning (or when necessary) to have space to hold the partial results; for instance:

If you practice the previous example together with the two traditional exercises: 898✕989, using 898 both as multiplier and multiplicand, you will be prepared for any traditional multiplication problem.


== The 123456789 exercise in multiplication ==


== References ==


== Further reading ==
Kojima, Takashi (1963), "III Other multiplication methods", Advanced Abacus: Theory and Practice, Tokyo: Charles E. Tuttle Co., Inc., ISBN 978-0-8048-0003-7 Totton Heffelfinger (2004). "Traditional Multiplication techniques for Chinese Suan Pan - The Extra Bead and the Suspended Bead". 算盤 Abacus: Mystery of the Bead. Archived from the original on August 1, 2021. https://web.archive.org/web/20210801083036/http://totton.idirect.com/suanpan/. Totton Heffelfinger (2013). "Suan Pan and the Unit Rod - Multiplication". 算盤 Abacus: Mystery of the Bead. Archived from the original on August 1, 2021. https://web.archive.org/web/20210801083439/http://totton.idirect.com/suanpan/shiftMultiplication/.