Raising an all-in bet -- Rule clarification

[ElSapo]

Apologies in advance b/c I know this is in the archives, but my searches, depending on what I search, turn up altrenatively 18,000 posts and none.

In limit (or no limit), if a player bets, calls or raises all-in for less than half a bet, can subsequent players raise this? If it is exactly half a bet? More than half?

Thanks.

El Sapo