The objective of Qix is to fence off, or “claim”, a supermajority of the playfield. At the start of each level, the playing field is a large, empty rectangle, containing the Qix—a sticklike entity that performs graceful, but unpredictable motions within the confines of the rectangle.
The player controls a small diamond-shaped marker that can move around the edges of the rectangle.
To claim an area of the screen, the player must detach from an edge and draw Stix within the rectangle. Stix are simply lines traced out by moving the marker. When the marker completes a closed shape, the captured area becomes solid and points are awarded.
The player may draw either Fast Stix, which appear blue when solid, or Slow Stix, which appear red. Slow Stix take longer to draw, but are worth twice as many points. The player is vulnerable and can lose a life when drawing Stix if the Qix touches a Stix line as it is being drawn, before it becomes solid. While drawing Slow Stix, it is possible to switch to Fast Stix, but doing so will forfeit the doubled points.
Once the player has claimed an area, the marker can safely move along the border of that claimed area but must avoid “Sparx” which traverse the edges and will cost the player a life if touched.
There is a Time Meter located at the top of the screen. When the time meter reaches zero, two additional “Sparx” enter the playfield and continue to traverse the perimeter of unclaimed space. When the time meter reaches zero once again, the Sparx become Super-Sparx, having the ability to chase the player even up an unfinished Stix line. Neither the Qix nor the Sparx can be destroyed, only outmaneuvered.
To complete a level, the player must claim most of the playfield. (The game was shipped at 75 percent, but the arcade operator could adjust the requirement between 50 percent and 90 percent.) Players meeting the requirement won the round and earned a bonus of 1,000 points per additional percentage claimed (e.g., claiming 80 percent of the field won a 5,000-point bonus, since it was 5 percent more than needed).
Once there are two Qixes, drawing a complete line such that the Qixes are separated results in a “Splix Qix” and a multiplier applied to all future points earned for each Splix.
After the player completes two levels, the difficulty increases. Initially, two Qixes appear instead of one. From the third level on, Qixes move faster. On later levels, Sparxes move faster and become more plentiful. Eventually, only Super Sparxes appear.
There are three enemies in the game, each of which is fatal to the player.
1. The primary enemy is the Qix itself. If the Qix touches any part of an uncompleted Stix, the player loses a life. The Qix is dangerous only while a Stix is being drawn; it cannot hurt the player while he is simply traversing the unclaimed perimeter. The Qix moves faster and more aggressively at each higher level. Beginning at Level 3, there are two Qixes rather than one, greatly increasing the challenge. (In a two-Qix level, the player may successfully complete a level without filling 75% of the screen by drawing a Stix between the two Qixes, isolating them into separate sides of the game board. The score for the next level is doubled when this happens.) The Qix's strategy of random movements followed by lightning attacks, and its ability to coordinate its attacks with the position of the Sparx, gives the illusion of a highly intelligent adversary. Though primitive, this artificial intelligence makes Qix one of the most difficult arcade games — impossible to beat and only slightly less impossible to master.
2. Sparx are small red sparks that travel along the edges of the rectangle and of any claimed area. They have no intelligence and simply follow the path blindly at a constant speed. If a Sparx touches the marker, the player loses a life. Initially the Sparx cannot travel along the Stix that's currently being drawn, but if the player takes too long on one screen and/or at higher levels, the Sparx become blue Super Sparx, gaining the ability to follow the player up an unfinished Stix line.
3. The Fuse appears if the marker stops moving whilst in the process of drawing Stix. Fuses are similar to Sparx, but they travel along the Stix, appearing to 'burn' it in the manner of a real fuse wire. If the Fuse travels all the way to the marker, the player loses a life. The Fuse disappears as soon as the marker starts moving again.
4. The arcade preview screen cites a fourth enemy: the Spiral Death Trap. However, this arises only when the player has blundered while drawing a Stix. The marker cannot cross an unfinished Stix line, nor can it retrace its path, so it is possible for the player to draw a Stix that cannot be completed — in effect, painting himself into a corner. Death by Fuse then cannot be avoided (unless the Qix or a Super Sparx gets to the player first.)
In 2000, a port for the Game Boy Color was released called Qix Adventure. This version features a new "Adventure" mode where the player travels a map screen, taking on various opponents which appear on the playing field. This adds a level of difficulty to the game, as these characters must also be captured in order to complete each level.
As of 2007, the original game was available on the online game-playing service Gametap.
Other modifications include: