What is a 1:1 project screen?

In 1:1 project screen aspect ratio will be 1:1 by definition.

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Square-shaped screens used for overhead projectors sometimes double as projection screens for digital projectors in meeting rooms, where space is scarce and multiple screens can seem redundant. These screens have an aspect ratio of 1:1 by definition. Other popular aspect ratios include 4:3 and a widescreen ratio of 16:9, which are often used as dedicated data projection and home cinema use, respectively.
read more : http://en.wikipedia.org/wiki/Projection_screen

1:1 ratio provides a squared screen format for projectors.
This is mostly used in Classrooms.
This is also used for presentation.

The aspect ratio is 1:1. This ratio is common and frequently used ratio.