A few questions that come up often. If yours is not here, write to us — the form below, or plain email to contact@chartkeys.app.
Not required, but recommended for playing more than two notes at once, not to mention both hands. The onscreen keyboard plays everything if you are on the go, and the stats work the same way. If you do have a MIDI keyboard, connect it by cable or over Bluetooth. ChartKeys picks up wired MIDI automatically. You may need to go to the menu in the upper right corner to connect the Bluetooth MIDI keyboard. For wired USB MIDI keyboard, you might need a "camera connection kit".
iPhone, iPad, Mac-as-iPad. There is no Android, Windows or web version, and none is planned right now.
The reason most likely is the Bluetooth headphones. They are just too laggy for this. Try the app without anything connected first: just the phone speaker and the screen keyboard. That should sync well. Then try to connect things one by one, see if anything causes lag. Add wired headphones if you want them, those are usually low latency. Connect MIDI keyboard. Wired is best, Bluetooth for input is also okay, because it sends short signal codes not sound, and does not require buffering to be stable.
Yes, with Pro. Copy the song chart link to the clipboard and tap the import button in the Pro toolbar to add it into your songbook — the common irealb:// format is supported, so thousands of charts shared online work as they are. Playlist links also work. You can export your own charts the same way.
Pro is a one-time purchase tied to your Apple Account, so it follows you to every device signed in with it. Install ChartKeys there and tap Restore Purchases on the Pro screen. There is nothing to pay twice, and no account to sign into.
Your device's email app opens with the message ready to send. Telling us your device and iOS version helps a lot with anything that looks like a bug.