Another way to mitigate the risk of runway incursions is to  write down all taxi instructions as soon as they are received  from ATC.  [Figure 14-50]  It is also helpful to monitor ATC  clearances and instructions that are issued to other aircraft.  You should be especially vigilant if another aircraft has a  similar sounding call sign so there is no mistake about who  ATC is contacting or to whom they are giving instructions  and clearances.  Read back your complete ATC clearance with your aircraft  call sign. This gives ATC the opportunity to clarify any  misunderstandings and ensure that instructions were given to  the correct aircraft. If, at any time, there is uncertainty about  any ATC instructions or clearances, ask ATC to “say again”  or ask for progressive taxi instructions.  The most important sign and marking on the airport is the  hold sign and hold marking. These are located on a stub  taxiway leading directly to a runway. They depict the holding  position or the location where the aircraft is to stop so as not to  enter the runway environment.  [Figure 14-51]  For example,  Figure 14-52  shows the holding position sign and marking  for Runway 13 and Runway 31.  When ATC issues a “ hold short ” clearance, you are  expected to taxi up to, but not cross any part of the runway  holding marking. At a towered airport, runway hold  markings should never be crossed without explicit ATC  instructions. Do not enter a runway at a towered airport  unless instructions are given from ATC to cross, takeoff  from, or “line up and wait” on that specific runway.  Since inattention and confusion often are factors contributing to  runway incursion, it is important to remain extremely cautious  and maintain situational awareness (SA). When instructed to  use a runway as a taxiway, do not become confused and take  off on the runway you are using as a taxiway.  ATC is required to obtain a read-back from the pilot of  all runway “ hold short ” instructions. Therefore, you  must read back the entire clearance and “ hold short ”  instruction, to include runway identifier and your call sign.  Title 14 of the Code of Federal Regulations (14 CFR) part  91, section 91.123 requires you to follow all ATC clearances  and instructions. Request clarification if you are unsure of the  clearance or instruction to be followed. If you are unfamiliar  with the airport or unsure of a taxi route, ask ATC for a  “progressive taxi.” Progressive taxi requires the controller  to provide step-by-step taxi instructions.  The final decision to act on ATC’s instruction rests with you.  If you cannot safely comply with any of ATC’s instructions,  inform them immediately by using the word “UNABLE.”  There is nothing wrong with telling a controller that you are  unable to safely comply with the clearance.  Figure 14-50.  A sound practice is to write down taxi instructions  from ATC.  Figure 14-49.  Confusing runway/runway intersection.  ATC Instructions  14-32  ATC Instructions — “ Hold Short”  36