Structure
It is very common in English to put two nouns together without a preposition.

• tennis shoes    
• a sheepdog    
• the car door    
• orange juice

The first noun is like an adjective in some ways.

Compare:

• a race-horse ( = a sort of horse) a horse-race ( = a sort of race)
• a flower garden  ( = a sort of garden) a garden flower  ( = a sort of flower)
• milk chocolate    ( = something to eat) chocolate milk    ( = something to drink)

The first noun is usually singular in form, even if the meaning is plural.

• a shoe-shop    (NOT -a shoes shop)
• a bus-stop    (NOT a buses stop)

Some common short noun + noun expressions are written as one word (for example sheepdog). Others are written with a hyphen (for example horse-race) or separately (for example milk chocolate). There are no very clear rules, and we can often write an expression in more than one way. To find out what is correct in a particular case, look in a good dictionary.

Meaning
The first noun can modify the second in many different ways. It can say what the second is made of or from:

• milk chocolate    
  
• a glass bowl

or where it is:

• a table lamp    
  
• Oxford University

or when it happens:

• a daydream   
  
• afternoon tea

or what it is for:

• car keys    
  
• a conference room

Noun + noun + noun + noun ...

We can put three, four or more nouns in a group.

• road accident research centre ( = a centre for research into accidents on roads)

Newspaper headlines often have this structure.

Other structures

It is not always easy to know whether to use the noun + noun structure (for example the chair back) or the possessive structure (for example John's back). The rules are very complicated; experience will tell you which is the correct structure in a particular case.