'%' For instance '%d'

with each % indicating where one of the other (second, third, ...) arguments is to be substituted, and in what form it
is to be printed. For instance, %d specifies an integer argument

augment each %d

For Instance

printf("%3d'F%6d'C\n",Fahrenheit,Celsius);

to print the first number of each line in a field three digits wide, and the second in a field six digits wide,

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed at
least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number
(celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output looks
like this:

printf("%3.1f'F\t%6.1f'C\n",Fahrenheit,Celsius);

0.0'F      -17.8'C
5.0'F      -15.0'C
10.0'F     -12.2'C
15.0'F      -9.4'C

    printf("%3.0f'F\t%6.1f'C\n",Fahrenheit,Celsius);

  'F     -17.8'C
  'F     -15.0'C
 'F     -12.2'C
 'F      -9.4'C

printf("%3.2f'F\t%6.3f'C\n",Fahrenheit,Celsius);

0.00'F    -17.778'C
5.00'F    -15.000'C
10.00'F    -12.222'C
15.00'F    -9.444'C
20.00'F    -6.667'C
25.00'F    -3.889'C
30.00'F    -1.111'C
35.00'F     1.667'C
40.00'F     4.444'C
45.00'F     7.222'C
50.00'F    10.000'C
55.00'F    12.778'C
60.00'F    15.556'C
65.00'F    18.333'C
70.00'F    21.111'C
75.00'F    23.889'C
80.00'F    26.667'C
85.00'F    29.444'C
90.00'F    32.222'C
95.00'F    35.000'C
100.00'F    37.778'C
105.00'F    40.556'C

Width and precision may be omitted from a specification:
 %6f says that the number is to be at least six characters
wide; 
%.2f specifies two characters after the decimal point, but the width is not constrained;
 and %f merely says
to print the number as floating point.

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s for character
string and %% for itself %.

uses %f for both float and double; %.0f suppresses the printing of the decimal point and the
fraction part, which is zero.