The stick used to hit billiard balls, called a billiards cue, is different in some ways from the typical pool cue. Compared with pool cues, billiard cues are often shorter, with a shorter end cap (called a ferrule), a fatter bottom portion where the back hand grips the stick (called a butt), a wooden screw in the middle rather than one of metal or plastic, and a smaller tip diameter. These features make the billiard cue stiffer. This stiffness helps players in striking the larger and heavier billiard balls as compared with pool balls. The stiffness also acts to reduce an effect called "deflection" (sometimes called "squirt"). Deflection is an undesirable effect from the use of sidespin. Sidespin is spin placed on a ball by striking it not at its center but off to one side of its center, causing it to spin as it travels down the table. Deflection causes a ball to travel not in a straight line in the direction it was struck.