When we read a sentence or a passage, we sometimes find the word like /postman/, /railway/, /screwdriver/, /nobleman/, /strong-hold/, /breakfast/, /walking-stick/, /income/, etc. Those words are called "Compound Words".
When we try to take notice of the words carefully, we will find that the words, actually, a combination between one word and another which creates a new meaning and, sometimes, is far from the root of the word.
Basically, Compound words may be formed from :
1. Noun + noun
e.g. armchair (arm + chair), shoemaker (shoe + maker), teaspoon (tea + spoon), railway (rail + way), etc
The word /armchair/ is formed from the noun /arm/ and the noun /chair/.
2. Adjective + Noun
e.g. sweetheart, halfpenny, shorthand, etc
Take notice that the word /sweet/ is adjective and the word /heart/ is a noun.
3. Verb + Noun
e.g. breakfast, pickpocket, hangman, telltale, daredevil, etc
4. Gerund + Noun
e.g. walking-stick, writing-book, writing-desk, drawing-room, etc
Note : The combinations are not the same as, for example, /singing bird/ or /sleeping child/.
/Singing bird/ is not gerund!. The word /singing/ is present participle. To understand it, you have to take notice of the meaning. The word /singing/ indicates one action is being done by a thing or a person. But the word /walking/ in /walking-stick/, we do not find that the stick is walking and, the fact is, the stick is used to walk.
In this case we can make conclusion that a compound word which is formed by the combination of gerund and Noun is that the Verb-ing must indicates the use or the function of the noun. And when the verb-ing indicates the action is being done, the word is not the compound word, but a phrase! (a phrase is two words or more which are combined to make a spesific meaning but can not stand alone because it doesn't contain the element of 'subject' and 'verb')
5. Adverb (or preposition) + Noun
e.g. outlaw, inside, afternoon, foresight, bypath, etc
6. Verb + adverb
e.g. die-hard, go-between, lock-up, look-up, send-off, etc
7. Adverb + verb
e.g. Income, outcome, outgo, overdue, overact, overcome, etc