Not flat at all

• Like good pianos
• Ready for the piano recital
• At the right pitch, musically: 2 wds
• Sounding fine
• Perfectly pitched, like a guitar's strings: 2 wds
• Like well-maintained pianos
• Ready for a concert
• Not off key
• Jibing

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Intune \In*tune"\, v. t. To intone. Cf. Entune .