sentences:  "Don't care" may also refer to an unknown value in a multi-valued logic system, in which case it may also be called an X value or don't know.In the Verilog hardware description language such values are denoted by the letter "X".In the VHDL hardware description language such values are denoted (in the standard logic package) by the letter "X" (forced unknown) or the letter "W" (weak unknown).