Newbie Q: How are blocks defined?

[ihsotas]

They don't seem to be similar in time-to-solve, number of transactions, total BTC sent, or block size. Is there any standard way in which blocks are defined while they are being solved, OR is anyone free to group the most recent transactions (coming after the last solved block) into a block and try to create the special zero-prefix SHA256 hash? So that I might be trying to solve a block with 137 transactions, but someone else is trying to solve a block with just the first 56 of those 137, leaving the remainder to be solved as part of another block?

[DannyHamilton]

They don't seem to be similar in time-to-solve, number of transactions, total BTC sent, or block size. Is there any standard way in which blocks are defined while they are being solved

https://en.bitcoin.it/wiki/Blocks

http://james.lab6.com/2012/01/12/bitcoin-285-bytes-that-changed-the-world/

is anyone free to group the most recent transactions (coming after the last solved block) into a block and try to create the special zero-prefix SHA256 hash? So that I might be trying to solve a block with 137 transactions, but someone else is trying to solve a block with just the first 56 of those 137, leaving the remainder to be solved as part of another block?

Yep.  As a matter of fact, you can even work on a block that has only 1 transaction (the coinbase transaction that pays you the block reward when you solve the block), and no others.  Here's an example:

http://blockchain.info/block-index/381211/00000000000000fded04b4c7e2c9936310f5642e2dd9a834187a8afa4eba14ec

[ihsotas]

Thanks, Danny. That lab6 article is very well written.