Have an ace/a trick up your sleeve - Idioms by The Free Dictionary
My trump card. (Words and phrases meaning 'advantage') – About Words – Cambridge Dictionary blog
Wild Cards: 7 ESL Card Games You've Gotta Have Up Your Sleeve | FluentU English Educator Blog
The Jazz Sax & Improvisation Blog of Saxophonist Bobby Stern - bobbysternjazz.com
Idioms through pictures - to have a card up one's sleeve to have an advantage that other people do not know about: England have definitely been the weaker side, but I think