UDCA

Upside Down Count and Attitude (UDCA) - Inverting the "standard" meanings and depending on whether count or attitude signals are appropriate:

Attitude signals:

A high-low signal shows negative attitude

A low-high signal shows a positive attitude

Examples: Partner leads a SK, obviously the top of a sequence. Partner signals:

Low-High shows interest in the suit, or no interest in switching to another suit

High-low shows disinterest in the suit, requesting leader to switch to another suit

Conditions for attitude signals:

One partner leads the suit, other partner follows playing to the trick

Either side leads a suit, partner is void, discarding in a side suit

When the value of providing attitude signals to partner exceeds the value to Declarer

Also see Smith Echo

Count signals:

High-low shows an odd number of cards in the suit

Low-high shows an even number of cards in the suit

Conditions for count signals:

Opponent leads a suit

One partner leads the suit, other partner follows playing to the trick (although most players show attitude signals in this scenario)

When the value of providing count signals to partner exceeds the value to declarer