A fluid ejection system can include a carriage base that includes a first pen slide at a bottom rear of the carriage base, a second pen slide at a front of the carriage base, and a pen guide at a top rear of the carriage base, the pen guide can include a guide portion to engage a substantially vertical portion of a back corner of a pen, a blocker portion to engage a substantially horizontal portion of the top back corner of the pen, and a pen guide corner formed between the guide portion and the blocker portion to hold a top back corner of a pen substantially in place during an angular loading of the pen.