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Frontside and backside are surfing, skateboarding, snowboarding and aggressive inline skating terms that are used to describe how a person approaches an obstacle or performs a certain trick. In Aggressive Skating, frontside and backside are types of grinds.
The names frontside and backside originate from surfing where they mean the direction the surfer is facing while surfing a wave. If the surfer is facing the wave, he or she is surfing frontside, otherwise he or she is surfing backside. The terms forehand and backhand are synonyms for frontside and backside but they are only used in surfing.
Skateboarding
For performing rotational tricks on flat ground, frontside means that for the first 90-180 degrees of the rotation, the skater can see ahead in the direction of travel. For example, a regular frontside 180 Ollie is rotated toward the heels or anti-clockwise (clockwise for goofy skaters). Conversely, a regular Backside 180 is rotated toward the toes or clockwise (anti-clockwise for goofy skaters). When riding fakie, a trick is considered frontside if it is done with the same motion that would be frontside if the person were facing forward to start.
For grinds, Frontside means that the skater is facing the rail/ledge during the approach. A Backside grind means that the skater is facing away from the rail/ledge during the approach.
Snowboarding
In snowboarding, frontside and backside have the same meanings as in skateboarding. When turning, backside is analogous to a toeside turn, and frontside is analogous to a heelside turn.
In the air, backside means that you turn the front of your body into the rotation first and frontside means you turn your back into the rotation first. For example, a left foot forward (or regular) rider would rotate counterclockwise to do a frontside 360 and clockwise to do a backside 360.
Aggressive Inline Skating
In Aggressive Inline Skating, frontside and backside are used to describe the way the skater's feet are positioned when grinding on an object. For example, if a skater jumps on to grind a ledge with both feet landing on the h-blocks (the grindable area in the middle of the frame), with the toes facing the ledge, the skater is doing a frontside grind. Their whole body will be facing the object.
A backside is much the same, although when the skater jumps on, they will be landing with their back to the object, and with their heels facing the ledge.