Generally, when a compound modifier comes before the word it modifies, you need to use a hyphen in the compound (e.g., a high-impact advertisement or a well-made handbag). Compound words formed from comparatives or superlatives also need hyphens.
When using high or low (or other adjectives) as part of a compound adjective before a noun, a hyphen should be inserted between high or low and the word that it modifies.
Some examples of compound adjectives using high and low are high-level/low-level, high-income/low-income, and high-impact/low-impact.
Consider the examples below for illustration:
However, when the compound comes after the noun in refers to, you should leave the hyphen out.
Comparative and superlative adjectives in compound words should also have a hyphen when they come before a noun.
There are a lot of modifiers used to create compound adjectives. Other modifiers commonly used for this purpose include well-, ill-, better-, best-, little-, and lesser-. Consider the following examples:
But remember to omit the hyphen when the compound comes after a noun.