Step 1: Legal trigger.
If a shareholder fails to pay the amount due on allotment/calls, the Board—after serving a proper notice—may forfeit the shares as per Articles.
Step 2: Purpose/effect.
Forfeiture cancels the membership and amounts already received are transferred to Share Forfeiture A/c (capital profit), enabling reissue of those shares. It enforces payment discipline and protects existing shareholders.
Final Answer:
\[
\boxed{\text{Non-payment of allotment/call dues (after notice) leading to cancellation of shares}}
\]
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