Use 'let alone' to highlight a progression from something improbable to even more so.

Increasing Improbability

'Let alone' emphasizes contrast, not a comparison for similarities.

Contrast Stressed

The phrase often points to extreme differences between two elements.

Illustrates a scale from what's already unlikely to what's even less likely.

Sometimes 'let alone' signifies no chance when placed after something implausible.

'Let alone' can express frustration over a situation not meeting expectations.