The unprecedented x-ray peak power available at the Linac Coherent Light Source (LCLS) makes possible, for the first time, experiments in which a sample is excited by x-rays, then probed with a delayed x-ray pulse as the excitation relaxes on the femtosecond timescale. A mirror-based x-ray split and delay (XRSD) instrument under construction at the LCLS will enable forefront science through time-resolved study of fundamental ultrafast x-ray matter interactions in the 500-2000 eV photon energy range. The XRSD will add x-ray pump/x-ray probe capability to the AMO and SXR beamlines, allowing experiments expected to have an impact on basic energy sciences research in areas such as atomic and molecular science, chemical physics, nanoscience, ultrafast science, and imaging. The XRSD splits the LCLS x-ray pulse into two portions by wavefront division, delays one by a user-selected time 0-100 femtoseconds (±0.1 fs,) then recombines them at a sample for x-ray pump/x-ray probe experiments on the AMO and SXR beamlines. Two mirrors located immediately after the AMO Kirkpatrick-Baez mirrors will split the FEL wavefront; one of these mirrors will be translated and rotated to produce a variable delay. The energy of both pulses can be independently measured on each shot, aiding data analysis. The XRSD will be integrated into the AMO/SXR control system and data acquisition system for user control. After commissioning, the device will be available for LCLS user proposals at the AMO and SXR endstations.