Blinds that are partially cut away give the horse better visibility downwards and forwards.  This makes it easier for horses to negotiate uneven ground and obstacles.
This design allows the horse to have more peripheral vision than a standard blind, but still prevents them from seeing behind.

 Minimum Length: 24cm (9") Maximum Length: 31cm (12") Cheek Width: 13mm (1/2")   Pony

Minimum Length: 27cm (11") Maximum Length: 35cm (14") Cheek Width: 19mm (3/4") Cob

 Minimum Length: 30cm (12") Maximum Length: 38cm (15") Cheek Width: 19mm (3/4") Full Size