The following terms are important in relation to D-slide valve :

 a) Steam lap or outside lap :- It is distance by which the outer edge of the D-slide valve overlaps the steam port. 

Note : In actual practice, the displacement of the D-slide valve is greater than the steam lap by a distance known as lead of the valve. 

b)  Exhaust lap or inside lap :- It is the distance by which the inner edge of the D-slide valve overlaps the steam port. 

c)  Valve travel :- It is the distance moved by the valve from one end to the other end. 

Note : The eccentricity  or throw of eccentric is equal to half of the valve travel.