1https://bitcoin.stackexchange.com/questions/42634/can-unupgraded-nodes-that-accept-zero-confirmation-be-fooled-after-segwit/42642 – G. Maxwell – 2019-01-21T16:19:09.647

@G.Maxwell "Among old nodes, the transaction is not propagated because it is non-standard", but how these tx's are accepted as valid by old nodes when inside a mined block? – Pitchas – 2019-01-21T16:29:32.330

If a pre-segwit miner would include a segwit tx in a block (which they won't accidentally do, due to nonstandardness), that block will be invalid to segwit miners. The latter would just ignore that block and build a chain that does not include it. – Pieter Wuille – 2019-01-21T16:34:36.743

@PieterWuille, this I understand, but what about the other way around? Do pre-segwit miners receive blocks by segwit miners with segwit tx's in them and accept them as valid? Being "nonstandard" and "valid" are different things? – Pitchas – 2019-01-21T16:39:36.817

2Nonstandard means "won't relay or include in a block" (which is a local policy). Invalid means "would be invalid in a block" (which is a network wide rule). Segwit has been constructed such that a block with witness transactions will be valid to pre-segwit nodes. – Pieter Wuille – 2019-01-21T16:42:48.447

@PieterWuille , perfectly clear now. Thank you for the attention and patience – Pitchas – 2019-01-21T16:47:16.347