A normal piano is an acoustic instrument with strings, set in vibrating motion by physical felt hammers striking them. Horizontal pianos (grand pianos) were first built in the mid 1700s and replaced the harpsichord as the main keyboard instrument is European music. Vertical pianos (uprights) were first made in the late 1700. Both grand pianos and upright pianos were being improved constantly throughout the 1800s. Range, volume, stability, all expanded and improved greatly during this period. By the 1900s, the design was set and not much has changed since then. When most people today say "electric piano" they usually mean an "electronic piano", and not an electrically amplified acoustic piano (like a Yamaha CP-80 from the 1970s). Digital pianos are modern electronic instruments designed to replicate the playing experience of an acoustic piano, in a portable, versatile instrument. A digital piano is a subset within the category of electronic keyboard instruments, which includes analog synthesizers, digital synthesizers, MIDI controllers, arranger workstations and more. Digital pianos have developed greatly in the last 40 years due to advancements in computer processing power, memory storage, and firmware. 20 years ago, digital pianos were mostly playback devices of pre-recorded samples, however, beginning about a decade ago, they started also using acoustic modelling algorithms and sample-based synthesis to create more acoustically realistic sounds complete with sympathetic string resonance responses, and mechanical movement noises.