There are 3 forms of compound words: the closed form, in which the words are melded together, such as firefly, secondhand, softball, childlike, crosstown, redhead, keyboard, makeup, notebook; the hyphenated form, such as daughter-in-law, master-at-arms, over-the-counter, six-pack, six-year-old, mass-produced; and the open form, such as post office, real estate, middle class, full moon, half sister, attorney general. How a word modified by an adjective — "a little school," "the yellow butter" — is different from a compound word — " a high school," "the peanut butter" — has clearly has something to do with the degree to which the preceding word changes the essential character of the noun, the degree to which the modifier and the noun are inseparable. If you were diagramming a sentence with a compound word, you would probably keep the words together, on the same horizontal line.

a dog house = a dog's kennel a house dog= a dog that stays inside the house Flower pot= a pot that has flowers planted in it a Pot flower does not make sense, you say a potted plant you say: 3, 50-cent coins a class 3 hours long = a 3-hour class another example 1.- a class that teaches sewing = A sewing class is correct 2.- a ticket for parking = a parking ticket is correct