Fortran stands for **For**mula **Tran**slating System - a programming language developed in 1857 at IBM.

program hello_world.f90 
```
program first 
	implicit none 
	! write 'Hello World!' to stdout
	write(*,*)"Hello world!"
end program first
```

compile and run with: 
```
$ gfortran hello_world.f90 -o hello_world
$ ./hello_world
```

All code taked place between  the program first and end program first statements, where first is the nmae given to the program. The `!` indicates a comment and the `write(*,*)` instructs the computer to write the statement Hello World to the screen, indicated  by the first asterisk using free-format, the second asterisk. The implicit none if a very important statement as well. 


```
program temperature 
	implicit none 
! declare variables 
	real :: DegC, DegF

	write(*,*)"Please type in temp in Celcius"
	! the read statement read input from 
	! keyboard and stores it invariable DegC 

	DegF = (9./5)*DegC + 32.

	! the write statement accepts multiple 
	! strings or variables, separated 
	! by commons
```