Transurethral catheters (Foley catheters) have one-way, two-way or three-way variations. The one-way catheter is a simple tube used to remove urine without securing the tube. This is used when someone is "straight catheterized." The two-way tubing has an outlet for urine and a conduit for water inflation of the catheter's balloon. This is the most commonly used catheter. The three-way tubing has an outlet for urine, an inlet for water inflation of the balloon, and an inlet for normal saline used for irrigating the urinary system most commonly for gross hematuria. There also exist whistle-tipped and multi-eyed variations. These tubes have larger and more draining holes, respectively, and are useful when blood clots are present. Most catheters are made of silicon, which is smoother than latex. Silicon catheters should be used in patients with an allergy to latex.