Hi,

I've just wanted to share with you iPhone application which I found very useful to quickly find chord names by the keys or to find the list of scales by its name or scale compatible chords:
https://itunes.apple.com/us/app/piano-companion-chords-scales/id586097063?ls=1&mt=8

This application is an iPhone version of popular Piano Companion for Android (see link: https://play.google.com/store/apps/details?id=com.binitex.pianochords)

Here you are full description of application:

Piano Companion is a flexible chord/scale dictionary with user libraries and reverse mode. Piano Companion allows you to find a chord or scale by its name. If you don't remember what's the name of a chord/scale, it helps you to find it by keys. Piano Companion shows chord/scale notes, degrees, fingering for both hands in major and minor scales. You can listen how a scale or chord sounds. You can see the list of compatible chords in the selected scale. The great advantage of Piano Companion is the fact that it's a growing application, so we will be glad to consider any new features which helps you to increase usability.

Key features:
- 1500+ piano chords including 3rd inversions
- 500+ scales
- 12 root keys
- Key notations: English, Italian, Japanese, Russian, etc.
- reverse chord/scale lookup
- ability to display chord on the staff
- audio support
- relative scale suggestions
- compatible scale chords
- scale lookup with major/minor scale fingering
- flexible chord/scale search using keyboard
- user libraries
- ability to create custom chords for a user library

Hope you will find it helpful!

This app doesn't seem to be available in the App Store for the iPad. Does anyone know if it works on an iPad?

Yes, new version works for iPhone/iPod/iPad. It also contains circle of fifths. Enjoy!