It is right to say all geometrical isomers are diastereomers.

It is NOT right to say that all diastereomers are geometrical isomers

Geometrical isomerism is just another name for cis-trans isomerism that can occur when two groups may differ in their orientation about a small ring or pi bond.

It would not be correct to call (R,R) tartaric acid a geometrical isomer of (R,S) tartaric acid since there is no double bond or ring.

Geometrical isomers are stereoisomers that are not mirror images, therefore diastereomers.

The whole term “geometrical isomers” could be replaced with “diastereomers” with very little loss, except people like to say, “cis-trans isomers” and the term “geometrical isomers” was devised as a slightly more official sounding name (which also covers E/Z isomers)

Fixed. Thank you!

That’s a great exercise for you to work on! Start by drawing cyclobutane, and then making all the chlorines “wedges” (or dashes, you’ll get the same thing). Then, systematically “flip” the dashes into wedges and compare to see if they are different isomers.