Can a block point to two prev blocks?

What does a fork mean?

Over time, one will win

Figure 1

Validation Rules

Figure 2

More on Soft Forks

Figure 5

Figure 6 and Soft forks

Soft forks

Invalid blocks

Hard forks

Who controls forks?

Soft fork doesn’t obtain > 50% of hash rate

Depends on the soft fork!

If old-rule blocks are now invalid

Hard fork does obtain > 50% of the hash rate

SPV wallets and forks

Soft and Hard forks in practice

Ethereum DAO hard fork

Summary