A focused grid consists of lead foil strips interspersed with strips of radiolucent material that are angled in order to coincide with the shape of the beam.

(gf) The grid frequency is the number of lead strips per inch or centimeter. An increased grid frequency involves thinner stripes. The most used grid frequency is 85 -103 lines per inch.

The grid efficiency or grid selectivity describes the efficiency in % of the primary beam transmission / % of transmitted scatter. The absorption of the primary beam is around 20 - 40%.

(B) The Bucky factor is the ratio of x-rays arriving at the grid (incident radiation), and those being transmitted through the grid. The Bucky factor describes how much the output of the x-ray tube must be increased in order to compensate for the removal of incident radiation by a grid.
B = incident radiation / transmitted radiation
For example, if for every two x-rays impinging the grid, only one will pass through, then the grid has a Bucky factor of two and the intensity of the x-ray beam must be doubled.