- T4XX = 14", T5XX = 15", X2XX = 12"

- The middle number is the last number of the year (T460/X260: 2016, T470/X270: 2017 etc.).

- The T-series has had three models per year, the normal with no ending letters that has an external battery, allows you to replace parts and is mainly meant as a durable corporate workhorse. The s-version is more for consumers, with usually better choices for panels and building material, but more integrated parts and less amount of maximum available memory; no replaceable external battery. The p-version means performance, so the laptop is bulkier but holds a faster 35W CPU.

- P-series are workstations. The s-model holds a ULV CPU, but the normal P51 and P71 can have Xeon/Quadro level parts. They're either 15" or 17" and for some mysterious reason annoy everybody with an off-center keyboard and a numpad.

- X1 Carbon is a 14" ultralight laptop with usually the best screens and best building materials.

- From the consumer models, the only thing that matters is the Yoga series. Don't have any experience from those, but people seem to like them.

[0] https://reddit.com/r/thinkpad