There are many different variations on the construction of and materials used in acoustic guitars. More expensive types feature solid wood tops (usually spruce or cedar), sides and backs (often rosewood, maple or mahogany).
Lower priced guitars can combine solid tops and laminated backs and/or sides. Entry-level acoustic guitars are usually made entirely of laminated woods. Necks are generally made of mahogany and fretboards are usually hardwoods, such as rosewood or ebony. The various combinations of the different woods and their quality, along with design and construction elements (for example, how the top is braced) are among the factors affecting the sound of the instrument. Many players and builders feel that a well made acoustic guitar's tone improves over time.
Since a steel-string guitar must withstand higher string tension than nylon-strung instruments, heavier construction is required overall. Steel-string guitars use different bracing systems to their classical counterparts, typically using x-bracing or ladder bracing instead of the fan bracing that is commonly found on classical and flamenco guitars.